Documentation

Why did my email bounce?

A bounce is the receiving server telling you it did not accept the message. The reason is in a status code and a sentence; this page translates both.

Hard and soft bounces

A hard (permanent) bounce means the message can never be delivered to this address as it is: it does not exist, or the receiver blocked you. Mailtruss adds the address to your suppression list so later sends to it are skipped until you remove it. A soft (temporary) bounce means a problem that may pass, such as a full mailbox or a busy server. Mailtruss's delivery network already retried for a while before reporting a soft bounce, and it does not retry again by itself.

The status code is the receiving server's enhanced SMTP status (RFC 3463): the first digit is 4 for temporary and 5 for permanent, and the rest says why. Mailtruss reads the delivery network's own bounce type first, then this code, then the server's text, and says so when none of them is specific instead of guessing a cause. Every bounced message's page in the dashboard shows the diagnosis and the raw evidence.

Bounce reasons

Recipient address does not exist

Type: Permanent
Typical code: 5.1.1, 5.1.0, 5.1.6
What it means: The receiving server says there is no mailbox for this address ("user unknown", "no such user", "mailbox unavailable").
Safe to retry?: No. Repeated hard bounces damage your sender reputation.
What to do: Remove or correct the address.
Full guide: 550 5.1.1: the recipient address does not exist

Recipient domain does not accept email

Type: Permanent
Typical code: 5.1.10
What it means: The recipient's domain publishes a null MX record, which means it accepts no mail at all.
Safe to retry?: No.
What to do: Correct the address; mail to this domain cannot be delivered.

Recipient domain could not be found

Type: Permanent
Typical code: 5.1.2
What it means: The recipient's domain has no mail server in DNS ("host not found", "no MX").
Safe to retry?: No, not until the address is corrected.
What to do: Check the domain part of the address for typos.
Full guide: 5.1.2: the recipient's domain could not be found

Recipient mailbox is full

Type: Either
Typical code: 5.2.2, 4.2.2
What it means: The receiving server accepted the address but the mailbox has no room. A 4.x code is temporary; a 5.x code is permanent.
Safe to retry?: Later, if the bounce was temporary. A permanent full-mailbox bounce usually means an abandoned mailbox.
What to do: Try again later, or reach the recipient another way.
Full guide: Mailbox full: 5.2.2 and 4.2.2

Message is too large for the receiving server

Type: Permanent
Typical code: 5.3.4, 5.2.3
What it means: The receiving server's size limit is lower than this message.
Safe to retry?: Only after you make the message smaller.
What to do: Shrink or remove attachments, or link to hosted files.
Full guide: 5.3.4: the message is too large for the receiving server

Your sending domain failed the receiver's authentication check

Type: Permanent
Typical code: 5.7.23, 5.7.25, 5.7.26, 5.7.27
What it means: The receiving server rejected the message because SPF, DKIM or DMARC did not pass for your sending domain.
Safe to retry?: Only after the cause is fixed.
What to do: Confirm DKIM and the return-path records are verified on the domain, and that your DMARC policy matches.
Full guide: Bounced for failed authentication: SPF, DKIM, DMARC

The receiving server blocked the message by policy

Type: Permanent
Typical code: 5.7.1, 5.7.0, 5.7.28
What it means: The receiver refused the message for a policy reason: spam filtering, a blocklist, or the recipient's own rules. The address itself may be valid.
Safe to retry?: Not until you know the reason.
What to do: Read the receiver's diagnostic text for its reason, and fix the cause before sending again.
Full guide: 550 5.7.1: blocked by the receiving server's policy

The receiving server rejected the content

Type: Permanent
Typical code: content-related 5.x codes
What it means: The message was refused because of its content, for example spam filtering or a policy on links or attachments.
Safe to retry?: Only after you change the message.
What to do: Check links, attachments and wording against the receiver's reason.

An attachment was rejected

Type: Permanent
Typical code: 5.x, attachment-related
What it means: The receiving server refused the message because of an attachment type or its contents.
Safe to retry?: Only without the attachment.
What to do: Remove the attachment or send a link to it instead.

The receiving side could not route the message

Type: Permanent
Typical code: 5.4.x
What it means: The message failed in transit, for example a mail loop or a misconfigured forwarder on the recipient's side. This is usually not a problem with your message.
Safe to retry?: It will fail the same way until the recipient's routing is fixed.
What to do: Contact the recipient another way.

Temporary delivery failure

Type: Temporary
Typical code: 4.x.x
What it means: The receiving server returned a temporary failure, for example a busy or unavailable server. The delivery network already retried for a while before reporting it; it does not retry again by itself.
Safe to retry?: Yes, later.
What to do: Send again later. If it keeps happening for one domain, the problem is likely on the receiver's side.
Full guide: Temporary delivery failure: 4xx

Address is on the delivery network's suppression list

Type: Permanent
Typical code: none (the message was not attempted)
What it means: The message was not attempted: this address bounced or complained before, so the delivery network refused to send to it.
Safe to retry?: No.
What to do: Confirm the address is correct and the recipient wants your mail before removing it from the suppression list.
Full guide: The address is on a suppression list

The receiving server permanently refused the message

Type: Permanent
Typical code: other 5.x.x
What it means: The receiver did not give a specific reason Mailtruss can classify, and Mailtruss does not guess one.
Safe to retry?: Not until you know why it was refused.
What to do: Read the diagnostic text on the message's page.

Keeping bounces low

Only send to addresses people gave you, remove or fix the ones that hard-bounce, and verify your sending domain so authentication passes. Mailtruss watches the share of hard bounces and spam complaints per organization and acts at fixed levels: see the acceptable use policy. To see each outcome without sending real mail, send to bounced@mailtruss.test in test mode.