Couple of comments
Haven't had a chance to fully read the comments so apologies if I am repeating anyone.
I used to use Mailcleaner - www.mailcleaner.org as a free, fully customisable border protection with good forum support;
For Dynamic or Static IP, DNSExit can provide not only free tools to update the Dynamic aspect but for around US$20 a year, a backup MX/storage feature;
A lot of mail systems will bork at receiving mail from consumer IP addresses; Likewise a lot of ISP's like to close off known mail ports to force the use of business broadband.
Hope some of this is useful. :)