"mailer.example.com", I suppose; or do they name things differently up there? I hear "everything's up to date in K.C.".... 😃
Seriously, though; I run UNIX, so it's not an issue for me. In php.ini, under the heading "Module Settings", there are a couple of defines that are Windows-specific, and I expect you might have to enter a hostname there...Windows implementation of mail() uses socket calls to a defined (external[?] - I assume you definite as such, at least) host....