These emails are generated as a result of your ISP having a catchall mailbox that MDaemon’s DomainPOP (Setup -> Server Setting -> DomainPOP) collects from. As your ISP accepts <anything>@yourdomain.com and routes it into this mailbox, when MDaemon collects this mail it finds no local users to distribute these emails to. It therefore passes it to the postmaster alias as a “parsing notification failure” so that they can manually be checked to see if this message was mis-addressed.
As these emails have already been accepted by your ISP you can’t reliably bounce them. However, you can tell MDaemon to route them differently and ultimately delete them as follows…
1. In Setup -> Server Settings -> DomainPOP -> Processing, untick “Ignore unknown local addresses parsed from messages”:
2. In Setup -> Server Settings -> Servers & Delivery -> Unknown Mail, only tick “…sent to the bad message folder”:
3. Optionally, in Setup -> Preferences -> Disk 0> Disk Cleanup and Backup tick set a suitable value in “Delete bad queue files older than this many days” (this will mean there is no way to review these messages if they’ve been mis-addressed and are not reviewed before the number of days you specify):
Please note: Although this will prevent this email reaching the postmaster address, we would strongly recommend switching from DomainPOP mail collection to direct SMTP delivery so that sender mail servers deliver email directly to MDaemon. This ensures it is MDaemon that determines if the email being sent is addressed to a valid local address and it will actively refuse to accept those which aren’t. This means the sender would receive a bounce back and is therefore aware the email was not accepted and delivered to the intended recipient. This article details how you can switch from DomainPOP to SMTP.