Judging by http://forums.gentoo.org/viewtopic-t-898610-start-0.html, apparently
Windows Phone 7 requires that "login" auth be enabled to send messages via a
Postfix server (that's authenticating using Dovecot). I have no idea how I was
supposed to figure that one out...
# plain login digest-md5 cram-md5 ntlm rpa apop anonymous gssapi otp skey
# gss-spnego
# NOTE: See also disable_plaintext_auth setting.
-auth_mechanisms = plain gssapi
+auth_mechanisms = plain login gssapi
##
## Password and user databases