A mail transport agent is what actually sends the mail. If you can connect to outside servers on port 25, you can run one. In fact, this is one of the sorts of zombies that viruses will install on your machine, but then it is to send spam.

However, it is possible that your IP range (called a CIDR block) has been identified as as source of spam already, so some receivers may not listen to you. But it is worth a try.

Two common transport agents are Sendmail for UNIX and Exchange for Windows.