User accounts with "@"

I have read your notes regarding using '@' in user accounts, but as my users will be moved from an old system which uses this, I obviously would like to continue with such names.

If I understand correctly postfix will required 2 user entries in /etc/passwd for each user in order to support user@domain style accounts. I'm assuming virtualmin fully understands this requirement and will handle this when creating user accounts. Is this correct?

Is there anything else I'll need to configure to ensure this works properly? Does postfix and/or dovecot need any special configuration settings that I must make (or does virtualmin handle this entirely)?

Other than the less than ideal dual entry into /etc/passwd, is there any other downside to using user@domain accounts? You indicated that some other apps might not properly understand these additional entries in /etc/passwd, but at this point, I'm don't know of any outside of the virtualmin environment I'm planning on using.

This is a FreeBSD box, so any info on this would be helpful.

Thanks.