Limited support for the VRFY and EXPN commands, which are generally not implemented due to being considered security issues, would be somewhat nice.
Limited support meaning the commands are only available to:
VRFY could be used as a standard way for something external to the mail server to determine if a mailbox exists. Not too dissimilar to Finger / SYSSTAT, but via SMTP. EXPN could be useful to dump the contents of a mailing list, for debugging or other purposes, since there's not really another mechanism to do this.
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