Mr. Phil where are you? ;-)
Right here. Just let me get out my kazoo and play the "Mighty Mouse" fanfare... Geez, I must be getting some kind of reputation!
So you're using PHP's mail(), or are you using some other form of mailing library? If mail(), for the fourth argument you're giving "From:
xyz@mysite.com" style? If that doesn't override the default
AccountName@server.lunarwhatever.com, you might need to open a support ticket. It sounds like it knows the real email address (what you gave for "From:") shows up when replying? In what other places does the default email address show up, and does the correct email address show up?
Maybe you could show us a sample email that isn't working (dummy names and content, of course), along with the PHP code. I seem to vaguely recall hearing about this problem from time to time, not necessarily at LP. I trust that you've done an extensive search, both with the forum search and with Google? Maybe you're using mail() and your account is configured to use SMTPmail() or something, or vice-versa?