MailEnabled

At work we use a SMTP server product to allow our applications to send outgoing email without authentication (safe list by IP address).  The product we were using was purchased before I started working for them.  It has the basic functionality, but we needed a little more.  I ended up replacing it with MailEnabled, which has a totally free version, is very configurable and fast.  

If you need a good SMTP/POP3 server that does groups, multiple domains and will function as a list server, check it out.