Note to self more than anything - sendmail under Solaris comes configured by default to relay mail to mailhost.mydomain.com which is useful as that's usually what you want. FreeBSD on the otherhand doesn't, and tries to send the mail directly. If this isn't what you want, for instance if the machine doesn't have a direct internet connection, you need to configure the DS macro in /etc/mail/sendmail.cf thus :
DSrelay.domain.com
From here
So there you go.