I tried to search this one out as I read it yesterday, but was unable
to actually track down the source…
With my setup, I POP multiple accounts through Spam Intercepter, which pulls mail from 2 different accounts. I have the second (my .Mac) account disabled in Mail since the messages come through when I POP into SI. Because the account is disabled it is not that simple to initiate a new message from it without manually activating it each time… until now.
By simply adding the email addresses you want to use separated by a comma in the account config, (in the email address field) you can bring back the pull-down in Mail which lets you pick which address to use. Pretty cool and very simple.
BTW — I DO NOT HACK MAIL ACCOUNTS. I cannot offer you any password assistance.