Since i upgraded my MediaWiki installation to 1.18, it isn't able to send mails via my smtp relay anymore (MS Exchange Server), for example when creating an user (error message: 501 5.1.7 Invalid address). But it did work with 1.17 and before.
It works again when I change Line 59 of includes/UserMailer.php (second if clause in toString()) in a manner that forces the if clause to be false (included && 1 != 1). Therefore no user name appears in the from header and emails can be send.
I am aware that "wrong" FROM headers are a well-known problem (as seen in bug reports like https://bugzilla.wikimedia.org/show_bug.cgi?id=4979 or https://bugzilla.wikimedia.org/show_bug.cgi?id=5494) and i am sure there is a really huge chance it isn't mediawikis fault. ;-)
Email is configured in LocalSettings.php by $wgSMTP.