Username w/ domain addition for mail users only

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1
SYSTEM INFORMATION
OS type and version Deb 12
Webmin version 2.202
Virtualmin version 7.20.2 Pro
Webserver version Apache 2.4.61
Related packages N/A

Is there a way to have usernames append the domain name only when creating mail users?

This system is primarily a personal server with my business website and a few sandboxes for playing around. I ssh in a lot from my phone due to port restrictions at my “real” job and typing long usernames is tiring. If possible, I’d like to leave usernames as “avoid clash only” for system logins, but append the domain name for any manually created email accounts. Is this possible?

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As a mail user is also unix (linux) user I would guess not, why not just produce a unix (linux) user to allow access via a short username

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Old-School tendencies I guess. No worries, was just asking. Thx!