I create usernames that contain the target domain as part of the username, so that I have a system that is extensible and easily remembered.
1. Decide on a 'prefix' to use for each domain, eg "dom1" for "domain1.com", "dom2" for "domain2.com"
2. Create user accounts as "dom1.mary" for mary@domain1.com, "dom2.mary" for mary@domain2.com, etc.
* There's a length limit on usernames (14 chars, I believe), so keep your domain prefixes short.
* Use a neutral domain as your Primary domain, so that *everyone* has to change their webmail settings (if your Primary domain is 'domain1.com', then the default from & reply to addresses in webmail for 'dom2.mary' will be 'dom2.mary@domain1.com' - better for this to be 'dom2.mary@myisp.com')
I don't know what happens to the <firstname>_<lastname>@... pseudonyms if you have identical "John Smith" accounts in different domains - you may be prevented from creating the second "John Smith" account, or the results may be undefined.