Well, the Campaign Manager does not interface with your email program at all. The Campaign Manager uses the Maximixer Email Service, which connects directly to your SMTP server, completely bypassing your email program.
If you're using Enterprise 7.5 or earlier, the Maximizer Email Service does not provide for SMTP authentication, so if your ISP requires you to send a username/password when sending an outgoing email, this may be the cause of the problems. Enterprise 8 supports SMTP authentication.