Marc,
In order for SmarterMail to sign the message, the client must first authenticate. On your client\server that's attempting to relay are you passing valid SMTP credentials ?
Regarding spam checks. I would recommend checking out Bruce's guide for further recommendations. This can be found here: http://portal.chicagonettech.com/kb/a171/smartermail-antispam-settings-document.aspx
Von See
Technical Support Supervisor
SmarterTools Inc.
(877) 357-6278
www.smartertools.com