5. Recommended Actions

The MSA SHOULD do all of the following:

5.1. Enforce Address Syntax

An MSA SHOULD reject messages with illegal syntax in a sender or recipient SMTP envelope address.

If the MSA examines or alters the message text in way, except to add trace header fields, it SHOULD reject messages with illegal address syntax in address header fields.

Reply code 501 is to be used to reject a MAIL or RCPT command that contains a detectably improper address.

When addresses are resolved after submission of the message body, reply code 554 (with a suitable enhanced status code from [SMTP-CODES]) is used after end-of-data, if the message contains invalid addresses in the header.

5.2. Log Errors

The MSA SHOULD log message errors, especially apparent misconfigurations of client software.

It can be very helpful to notify the administrator when problems are detected with local mail clients. This is another advantage of distinguishing submission from relay: system administrators might be interested in local configuration problems, but not in client problems at other sites.

Note that it is important to impose limits on such logging to prevent certain forms of denial of service (DoS) attacks.