So run through your Settings and make sure you have a valid email address vs. the admin@localhost.local
setup. Typically I'll run a sql script to update all of those in the settings to something like no-reply@valid-domain.com
.
Secondly, check the email template, it most likely has the admin@localhost.local
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.