So run through your Settings and make sure you have a valid email address vs. the
firstname.lastname@example.org setup. Typically I'll run a sql script to update all of those in the settings to something like
Secondly, check the email template, it most likely has the
email@example.com in the from address. You'll want to make sure that is corrected too.
Lastly, some webparts have from email addresses already defaulted to a
@localhost.local account too, so be sure to check those as well.