Reasons for undelivered emails

An email is not delivered when the recipient's mail server refuses to accept it. The server always returns an error code with a reason. A code starting with 4 means a temporary problem that usually goes away by itself, a code starting with 5 means a permanent rejection. Most often the cause is a non-existent address, a full mailbox, an overloaded server or an unauthenticated sender domain. In this overview you will find what the individual codes mean, what Quanda does with them and what you should do.

In a nutshell

1. Every undelivered email has a reason. In Quanda you will find it in the lower part of the campaign statistics page.
2. Codes starting with 4 are temporary. For code 421, Quanda tries to deliver the email again, 5 attempts in total, 5 hours apart. For codes 450, 451 and 452, it blacklists the address temporarily and returns it to your valid contacts automatically once the period is over.
3. Codes starting with 5 are permanent. Quanda blacklists the address and removes it from further mailings.
4. Gmail, Yahoo and Outlook.com now also reject emails from domains without SPF, DKIM and DMARC. That is no longer a fault of the address, but of the sender.
5. Email addresses age. According to ZeroBounce, roughly a quarter of a database stops working within a year.
6. Go through the list of undelivered addresses after every campaign, correct the typos and replace the addresses that no longer exist.

In short: what an undelivered email is, what types there are and what to do about it

• What it is: an email that the recipient's server refused to accept. It is also called a bounce.
• What types there are: temporary non-delivery (soft bounce, codes 421, 450, 451, 452) and permanent non-delivery (hard bounce, codes 501, 550, 551, 552, 553, 554).
• What Quanda does: it tries to deliver emails with code 421 again (5 attempts, 5 hours apart), it blacklists addresses with codes 450, 451 and 452 temporarily, and it blacklists permanently undelivered addresses for good and removes them from mailings.
• What is at stake: anyone who repeatedly sends to addresses that do not work loses sender reputation, and their emails start ending up in spam even for people who want them.

What has changed since the article was published: The overview came out in 2017 as a list of codes. Since then, Gmail and Yahoo (February 2024) and Microsoft (May 2025) have introduced mandatory domain authentication and reject non-compliant emails with new codes. Since December 2023, Google has been deleting personal accounts that have been inactive for more than two years. We have therefore added the new codes, an explanation of how to read an error message and steps for keeping undelivered emails to a minimum.

What an undelivered email is

An undelivered email is a message that the recipient's mail server refused to accept and returned to the sender with an error code. The common term for it is a bounce. In Quanda you will find it in the campaign statistics under the Not delivered value.

Temporary non-delivery (soft bounce) is a rejection whose cause passes with time, for example an overloaded server or a full mailbox. Permanent non-delivery (hard bounce) is a rejection that repeating will not change, typically because the address does not exist.

Not delivered is not the same as spam. When the server accepts a message, it counts as delivered, even if it ends up in the junk folder. An undelivered email never reached the recipient at all.

The list below contains only a limited selection of the reasons and codes that appear in Quanda when your email has not been delivered.

If you come across a reason we do not mention, write to us at support@onquanda.com and we will be happy to add it.

How to read the error code of an undelivered email

A server's error message has three parts: a three-digit reply code, an enhanced status code made of three numbers separated by dots, and a text. Each provider writes the text in its own way, while the numbers are defined by standards and mean the same everywhere.

An example of a Gmail message:

550 5.1.1 The email account that you tried to reach does not exist.

The middle number of the enhanced code tells you the most. It shows you where to look for the problem.

Source: RFC 5321 (SMTP reply codes) and RFC 3463 (enhanced mail system status codes).

Tip: Codes with the middle number 1 or 2 are dealt with in your contact database. Codes with the middle number 7 are dealt with on your side: through domain authentication, content, and who you send to and how often.

How to find out why an email was not delivered

In Quanda you will find the reason for non-delivery in two places. For a specific campaign in its statistics, for the whole account in the list of blacklisted contacts.

1. Open the statistics of the sent campaign and look at the Not delivered value.
2. In the lower part of the statistics page, go through the list of undelivered addresses. Each address has the reason for non-delivery next to it, meaning the error code and text returned by the recipient's server.
3. Use the first digit of the code to tell whether it is a temporary failure (digit 4) or a permanent failure (digit 5).
4. For a temporary failure, do nothing. For code 421, Quanda tries to deliver the email again, 5 attempts in total, 5 hours apart. For codes 450, 451 and 452, it blacklists the address temporarily and returns it to your valid contacts automatically once the period is over.
5. For a permanent failure, check the address for typos, correct it or replace the contact with a new one. You can delete an address that no longer exists.
6. Filter the contact list to Blacklisted contacts, export them to a CSV file and check the reasons for the whole account in the columns Hard Bounce reason and Soft Bounce reason.
7. If sender authentication, SPF, DKIM or DMARC keep appearing in the reasons, open Settings / Domain & DNS verification and check the DNS records of your sending domain.

When neither of the reason columns is filled in for a blacklisted contact, your email was marked as spam. It may have been marked by the recipient or by their email client. Do not contact such a contact again. More in the guide Blacklisted contacts, and the meaning of the other values is explained in the guide What the terms in statistics mean.

Temporary non-delivery: codes starting with 4

A message with a three-digit code starting with 4 usually appears when some transient error occurs while the message is being processed. These types of replies are usually no cause for concern, as they mostly sort themselves out with time.

Code 421: delivery temporarily deferred

Messages are temporarily deferred because of the rules or policies of the recipient's server. This mostly happens when too many messages or connections appear in too short a time.

Examples:

• 421 4.7.0 Message from (X.X.X.X) temporarily deferred
• 421 4.7.28 Gmail has detected an unusual rate of email. Email is temporarily rate limited
• 421 4.7.0 Message from (X.X.X.X) temporarily deferred, 4.16.50 (Yahoo)

How to handle this reply: You will see this type of message very rarely. After receiving it, we automatically schedule further sending of the email so that it is delivered in the shortest possible time. We try to deliver it again five times in total, 5 hours apart.

Watch out: Gmail also uses code 421 to slow down senders who do not meet its requirements. If you see SPF, DKIM, DMARC or "unauthenticated" in the text of the message, it is not about overload but about the authentication of your domain. Continue to the section on rejections because of domain authentication below.

Code 450: mailbox temporarily unavailable

The message was not delivered because the user's mailbox was unavailable. Probably because it was locked or could not be reached at that moment.

Examples:

• 450 too frequent connects from X.X.X.X, please try again later (throttled)
• 450 4.2.1 The user you are trying to contact is receiving mail too quickly. Please resend your message at a later time

How to handle this reply: No action is needed. We blacklist the address temporarily and return it to your valid contacts automatically once the period is over.

Servers that use greylisting also reply with code 450 or 451. Greylisting is a form of spam protection in which the server temporarily rejects the first attempt from an unknown sender and accepts the message only on a repeated attempt. A proper sending server tries again, most spam robots do not.

Code 451: temporary error on the recipient's server

This message was not delivered because of an error on the remote server. It is unlikely that the error is on your side.

Examples:

• 451 Resources temporarily unavailable. Please try again later [#4.16.1] (Yahoo)
• 451 Temporary local problem, please try again later

How to handle this reply: No action is needed. We blacklist the address temporarily and return it to your valid contacts automatically once the period is over.

Code 452: full mailbox or insufficient space

The message was deferred because of a lack of space on the server or in the recipient's mailbox. It is not your fault, and the message will probably be accepted later, once more space becomes available.

Examples:

• 452 Too many recipients at the same time (throttled)
• 452 4.3.1 Insufficient system storage (throttled)
• 452 4.2.2 The recipient's mailbox is full (Mail quota exceeded)

How to handle this reply: If your email is not delivered for these reasons, we blacklist the email address only temporarily. The time for which the address stays on the blacklist gradually gets longer with repeated unsuccessful attempts to deliver an email. The whole process is automatic and no action is needed on your side.

Permanent non-delivery: codes starting with 5

A message with a three-digit code starting with 5 usually means that some permanent error has occurred. The address may not exist, but delivery may just as well be blocked by the policies the recipient's server applies to the sender or to the content you are trying to deliver.

The same rule applies to all permanent codes. For email addresses that are not delivered for these reasons, we permanently set the blacklisting parameter to "Yes" and automatically remove them from further mailings. How a blacklisted address can come back is described in the section on blacklisting below.

Code 501: typo in the address

The email address you are sending to contains a typo or an invalid character. For example smith@qmail.com instead of smith@gmail.com, or an address with a space.

Examples:

• 501 Bad address syntax
• 501 5.1.3 Bad recipient address syntax

How to handle this reply: Correct the typo in the email address.

Code 550: mailbox does not exist or is blocked

The user's mailbox was unavailable or does not exist. Mostly because it was not found or because of the server's policy for incoming messages. Code 550 is the most common permanent code, and the enhanced code and the text after it always matter.

Examples:

• 550 Requested action not taken: mailbox unavailable. Host unknown
• 550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces
• 550 5.2.1 The email account that you tried to reach is inactive
• 550 Sorry, no mailbox here by that name (#5.1.1)
• 550 5.4.1 Delivery of the email was blocked on the recipient's side. Most often on suspicion of spam
• 550 5.7.1 Message rejected because of the recipient server's policy

How to handle this reply: For codes 5.1.1 and 5.2.1, check the address and correct or replace the contact. For codes 5.4.1 and 5.7.x, the address exists but the server rejected you as the sender. Domain authentication and a content check will help there.

Code 551: recipient is not on the server

The target mailbox does not exist on this recipient's server. The reply sometimes includes an address you can try sending the message to, if the server knows where the target mailbox is.

Examples:

• 551 not our customer
• 551 user does not exist

How to handle this reply: Ask the contact for their new address.

Code 552: full mailbox or message too large

The storage limit of the target mailbox has been reached, or the message exceeds the size the server accepts.

Examples:

• 552 5.2.2 This message is larger than the current system limit or the recipient's mailbox is full. Create a shorter message body or remove attachments and try sending it again
• 552 5.2.2 The recipient's inbox is out of storage space and inactive (Gmail)

How to handle this reply: If the code appears for several recipients, make the email smaller, the images above all. A mailbox that stays full for a long time tends to be an abandoned one, so try to verify the contact through another channel.

Code 553: invalid mailbox name

The message was rejected because the mailbox name was garbled or does not exist.

Examples:

• 553 5.7.1 Sorry, that domain isn't in my list of allowed rcpthosts
• 553 Invalid/inactive user

How to handle this reply: Check the address and correct or replace it.

Code 554: general rejection

The message was not delivered. This reply is an error report without a specific reason. In general it is a very vague message, so read the text after the code above all.

Examples:

• 554 Mail loop detected
• 554 5.1.0 Sender denied
• 554 5.4.7 The recipient does not have enough disk space for this message to be delivered
• 554 5.7.1 Mail rejected. Access denied. User does not exist
• 554 5.7.1 Mail rejected. Message blocked due to spam content

How to handle this reply: If the text talks about spam, policy or the sender, check your domain authentication and the content of the email. The article 4 basic rules to aviod having your e-mails sent to SPAM will help.

Rejections because of domain authentication: new Gmail, Yahoo and Outlook codes

Since 2024 there has been a new group of reasons that has nothing to do with the recipient's address at all. Mailbox providers reject emails from senders who do not have an authenticated domain. You can recognise them by the middle number 7 in the enhanced code and by the words SPF, DKIM, DMARC or "authentication" in the text of the message.

Source: Google, Gmail SMTP errors and codes; Microsoft, Fix error 550 5.7.515 in Outlook.com; Yahoo Sender Hub, SMTP Error Codes.

The key dates: Gmail and Yahoo introduced the requirements in February 2024, Microsoft did so for Outlook.com, Hotmail.com and Live.com on 5 May 2025, and since November 2025 Gmail has been temporarily and permanently rejecting non-compliant emails. Seznam.cz states in its rules for bulk senders that without valid SPF and DKIM records, messages may be delivered to the Spam folder or discarded altogether.

In Quanda, domain verification means three CNAME records and one TXT record for DMARC. You will find the whole procedure step by step in the article Gmail, Yahoo and Outlook requirements for bulk email senders and in the guide How to set up DNS records for sending.

Tip: These codes do not concern a single address but all recipients with a given provider. When most of your addresses at gmail.com or outlook.com suddenly fail, look for the cause in your domain authentication, not in your database.

Other reasons for non-delivery without a numeric code

Quanda shows a reason even when the recipient's server did not reply with any code or the connection was never made at all. Instead of a code you will then see a short text.

The best thing you can do is to leave addresses with a recurring reason out of your next mailings. You can also contact the owner of the mailbox to learn more about what may have happened.

Why addresses stop working

An email database is not a permanent asset. People change jobs, companies merge and close their domains, providers delete abandoned mailboxes. An address that worked last year may return 550 5.1.1 this year.

The most common causes:

• A change of job. In B2B this is the main reason. When a person leaves a company, their work address goes with them.
• Closed and inactive mailboxes. Since 1 December 2023, Google has been deleting personal accounts that have not been used for more than two years. This rule does not apply to work and school accounts.
• Typos made when entering the address. An address written down by hand at a trade fair or copied from a business card.
• Full mailboxes. A mailbox that stays full for a long time is mostly one that nobody reads any more.
• Purchased and inherited databases. Old addresses, addresses without consent and addresses set up as traps for spammers. Why you should avoid them is explained in the article Why purchased database stinks?.

ZeroBounce, a company that verifies email addresses, publishes every year how large a share of a database goes out of date within a year.

Roughly a quarter of a database every year is a lot. It means that a database you have not worked with for two years cannot simply be dusted off and sent a campaign.

What Quanda does with undelivered addresses

A blacklisted contact is a contact that is permanently or temporarily excluded from mailing because the last attempt to deliver an email to its address was unsuccessful.

On the recipient selection page, blacklisted addresses are shown in the red "Blacklisted" field once you select them. If you get this particular email address again in the future through an Active form, the blacklisting status is automatically set to "No" and your email campaigns will be sent to this address again.

Why does Quanda do this automatically? Anyone who repeatedly sends emails to addresses that do not exist looks to mail servers like a robot sending blindly. The servers start penalising the sender, and their emails then end up in spam even for recipients who want them. Blacklisting therefore protects the reputation of your domain.

How many undelivered emails are acceptable

The bounce rate is the share of undelivered emails out of all emails sent. You calculate it as Not delivered divided by Sent, times 100.

The thresholds are indicative and based on generally used rules of email marketing. Mailbox providers do not publish any binding threshold for undelivered emails, unlike the spam complaint rate, where Google states 0.3 %.

Google advises senders to reduce their sending volume when errors increase and to raise it again slowly. Seznam.cz explicitly asks senders in its policy to delete non-existent and inactive contacts from the database, to verify new contacts by confirming the address and not to use purchased databases.

How to reduce the number of undelivered emails

1. Collect addresses through a form, not by copying them. An address that a person enters in an Active form themselves is less likely to be written wrongly.
2. Check the result after an import. The Import results page shows the faulty records, including the line in the file, so you can correct them before you send a campaign to them. More in the guide Results of a bulk contact import.
3. Verify your sending domain. SPF, DKIM and DMARC remove the whole group of rejections with the middle number 7.
4. Send regularly. A database you send a campaign to once a year returns more errors than a database you write to every month.
5. Do not buy databases. According to Quanda's own analysis, which checked such databases with NeverBounce, their deliverability commonly drops below 70 %, and the damaged reputation carries over to the emails you send to your customers.
6. Clean your database continuously. Send one reactivation campaign to contacts who have not opened any email for half a year, and remove those who do not respond from your regular mailings. Contact scoring by open rate will help.
7. Spread out a large mailing after a long break. Start with the most active contacts and increase the volume gradually.

What to do after every campaign

• Look at the Not delivered value and calculate the bounce rate.
• Go through the list of undelivered addresses in the lower part of the statistics.
• Correct typos in the addresses.
• For permanently undelivered contacts, find out the new address or delete the contact.
• Check whether SPF, DKIM or DMARC keep appearing in the reasons.
• If undelivered emails jump at a single provider, check Settings / Domain & DNS verification.

How to read the other numbers in the campaign statistics is described in the article Email open rate: is 20 % a lot or a little?.

Final word

Is there a reason for non-delivery you do not understand? Have you suddenly had more undelivered emails? Contact us at support@onquanda.com or call +420 605 163 892. We will look at the reasons with you. The online course Email Deliverability from A to Z explains deliverability in context, from domain authentication to metrics. If you would like to go through the state of your database together, book a free consultation.