A bounce back email is an email which will be delivered to the sender's inbox after a sent message fails to deliver. The purpose of these messages is to alert the sender that something has happened to interrupt the email delivery and will provide additional detail to help in diagnosing the email issue and to understand why the message failed to deliver.


The main reasons for bounce back e-mails are as follows:

1. A non-existent email address - the recipient e-mail address is incorrect/does not exist

2. Undeliverable email - the recipient e-mail server is temporarily unavailable due to overload or maintenance.

3. Mailbox full - the recipient e-mail inbox is full

4. Blocked email - the recipient e-mail or sending e-mail has been blocked. This can occur if the sending e-mail has been intentionally blocked by the recipient or if either of the e-mail accounts have been blacklisted due to spam.

5. Malicious content - There are some specific types of files/content which we cannot send with an email.  e.g. sending some file types with a .exe or .wsf extension, as attachments with the emails may be blocked.

You can determine the severity of the failure on the first number for the status code contained in the bounced e-mail message content.

Status: 5.X.X -> means that it is a soft bounce.
Status: 4.X.X -> means that it is a hard bounce.


e.g.
This email has not been delivered to the recipient feedback@ezyvet.com. Full details from recipients mail service:
Action: failed
Final-Recipient: rfc822; feedback@ezyvet.com
Diagnostic-Code: smtp; 550 5.5.0 Requested action not taken: mailbox unavailable. [VE1EUR02FT011.eop-EUR02.prod.protection.outlook.com]
Status: 5.5.0
Diagnostic-Code: smtp; 550 5.5.0 Requested action not taken

Soft Bounce

There are many reasons for a soft bounce. A soft bounce will occur in situations where the recipient's inbox is full or their remote e-mail server is temporarily unavailable. In these cases, the sender’s mail server may be trying to resend the email to the remote mail server but will alert the sender about the issue so that the email can be resent at a later date.


Hard Bounce

A Hard bounce can occur if the recipient e-mail address is incorrect, or the message has been blocked. This is a permanent error and the message will not be resent. In this situation, you will need to contact the recipient to confirm their e-mail details before resending the message.




Additional technical details on the ezyVet e-mail servers can be found below:

The IPv4 address for the ezyVet mail server is dynamic which means it is not permanently set. Our servers are hosted on the Amazon server so the IPv4 address and any changes to this are handled by Amazon.


We use Amazon SES for all outgoing e-mails and send these to their endpoint, all communication from there is handled by Amazon. Sometimes email communication may be bounced due to blacklisting of our sending mail server. This can happen occasionally as some people send malicious content through SES and as we are all sharing the servers there can be short delays when amazon replaces the server and creates a new instance. This creates a new IPv4 address for us to unblock e-mails again moving forward.


Further technical detail can be found in the
Amazon SES documentation if necessary.