This article applies to:

• Trustwave MailMarshal (SEG)

Symptoms:

• Receiving a large number of Non Delivery Reports
• Error msg: 550 This message is from a non-existant email address
• Error msg: Sender Verify Failed

Causes:

This problem can happen when you have enabled the MailMarshal default rule 'Reject Messages with Blank Return Path' and you send email to a server that is performing a 'sender verify' on incoming email.

• Some email servers attempt to check the legitimacy of an email address by delivering an administrative email to your server, with a blank return path (no MAIL FROM address).
• If you have enabled the 'Reject Messages with Blank Return Path' rule, MailMarshal will refuse delivery of these administrative emails.
• The remote server then rejects delivery of the original email message you sent, because it cannot verify the sender.

Resolution:

• You can disable the default rule: 'Reject Messages with Blank Return Path'.

Notes:

The rule 'Reject Messages with Blank Return Path' is included in MailMarshal for use in exceptional cases where a storm of illegitimate NDRs is delivered to your server. This rule is disabled by default and it should not be left enabled for long periods. The following comment is attached to the rule: 

IMPORTANT - PLEASE READ:

This rule will reject all incoming messages where no return path is provided, I.E. where the connecting server issues a blank MAIL FROM address. However, enabling this rule will make you RFC non-compliant as you are obliged to accept blank return paths for the purposes of handling your own NDRs and returned messages.

Be aware that turning on this rule may also result in your server being included on "bad actor" block lists (you may be reported as non-compliant if you reject these messages).

See the following Trustwave Knowledgebase article for more information:

• Q10228 How do I block email where the SMTP "Mail From:" address is blank?