1. mfdnschecks

1.1. DESCRIPTION

This plugin is meant to be used on the MAIL command, and performs the following basic checks of the sender email address.
If the sender is empty, it is probably a bounce, so we have to accept it.
If it is not empty, it will require that the sender is of the form user@domain, both non-empty.
The domain must exist in DNS, and have either a MX or a A record. If it doesn't, that means
- there is no way to answer to the message, and/or the email address is madeup.
If there is a DNS error, it will postpone delivery by giving a temporary error. While not very effective, it will block some naive spammers, and will bounce messages with misspelt senders, which are always a support annoyance and a source of useless bounces.

Use of this plugin adds very little processor or memory load, but does add 1 or 2 DNS calls per message. That should not be a problem on any reasonable configuration, unless your server lacks Internet connectivity on delivery (for example if it uses fetchmail to get the mail and does the delivery while offline).

It logs every sender, accepted or not, so it does add 1 extra line of log per mail. Example logs:
- mfdnschecks: pid 7467 - OK sender@sending.com mfdnschecks: pid 23785 - blocked with: your envelope sender domain must exist: fakedomain.tld

If there is a DNS outage, checks can not be performed, and the message is temporarily rejected:
- 451 sorry, DNS temporary failure
If the sender's domain doesn't exist (or there is no way to send email to it), the message will be permanently rejected:
- 553 sorry, your envelope sender domain must exist: fakedomain.tld