It is not uncommon for your ISP to block incoming connections for servers, esp. mail servers. Likely the Terms of Service agreement you signed when getting the internet service forbid you from running at least some types of server applications, and more often than not a mail server on a residential connection is an infected machine spewing spam, malware, or both. As such, it's an easy way to limit the ability for spam masters/bot masters/etc. to expand their nets without check.