To remove an outdated formaly used e-mail address from the list taken into account when Apple Mail does autocompletion of e-mail addressss, open the list of previous adressees from the “Windows” menu, select the address in question an remove it from the list.

Using different sender e-mail addresses (aka “aliases”) for one(!) e-mail account in Apple Mail is as easy as that: Just list all addresses in the “Address” field of the account configuration form separated by commas. That all …

References: Article by Apple (archived, unmaintained)

OSX email