Qmail contains a known vulnerability which could potentially lead to massive abuse of the mail system and your server.
Qmail accepts E-mails addressed to individuals such as "administrator", but when Qmail recognises that the address is not valid, a non-delivery report is sent back to the sender of the E-mail.
In cases where a malicious users connects to your mail server, the owner of the random E-mail address may thousands of non-delivery reports.
If your mail server ends up being used for such a malicious attack, your account may be temporarily locked to prevent further abuse. We suggest to switch your mail server from Qmail to Postfix to avoid this vulnerability.