Any program that can send email elsewhere, by request from the network, can be used to transport spam. Spam is the usual name for unsolicited bulk email (UBE) or mass unsolicited email. It's also sometimes called unsolicited commercial email (UCE), though that name is misleading - not all spam is commercial. For a discussion of why spam is such a serious problem and more general discussion about it, see my essay at http://www.dwheeler.com/essays/stopspam.html, as well as http://mail-abuse.org/, http://spam.abuse.net/, CAUCE, and IETF RFC 2635. Spam receivers and intermediaries bear most of the cost of spam, while the spammer spends very little to send it. Therefore many people regard spam as a theft of service, not just some harmless activity, and that number increases as the amount of spam increases.
If your program can be used to generate email sent to others (such as a mail transfer agent, generator of data sent by email, or a mailing list manager), be sure to write your program to prevent its unauthorized use as a mail relay. A program should usually only allow legitimate authorized users to send email to others (e.g., those inside that company's mail server or those legitimately subscribed to the service). More information about this is in IETF RFC 2505 Also, if you manage a mailing list, make sure that it can enforce the rule that only subscribers can post to the list, and create a ``log in'' feature that will make it somewhat harder for spammers to subscribe, spam, and unsubscribe easily.
One way to more directly counter SPAM is to incorporate support for the MAPS (Mail Abuse Prevention System LLC) RBL (Realtime Blackhole List), which maintains in real-time a list of IP addresses where SPAM is known to originate. For more information, see http://mail-abuse.org/rbl/. Many current Mail Transfer Agents (MTAs) already support the RBL; see their websites for how to configure them. The usual way to use the RBL is to simply refuse to accept any requests from IP addresses in the blackhole list; this is harsh, but it solves the problem. Another similar service is the Open Relay Database (ORDB) at http://ordb.org, which identifies dynamically those sites that permit open email relays (open email relays are misconfigured email servers that allow spammers to send email through them). Another location for more information is SPEWS. I believe there are other similar services as well.
I suggest that many systems and programs, by default, enable spam blocking if they can send email on to others whose identity is under control of a remote user - and that includes MTAs. At the least, consider this. There are real problems with this suggestion, of course - you might (rarely) inhibit communication with a legitimate user. On the other hand, if you don't block spam, then it's likely that everyone else will blackhole your system (and thus ignore your emails). It's not a simple issue, because no matter what you do, some people will not allow you to send them email. And of course, how well do you trust the organization keeping up the real-time blackhole list - will they add truly innocent sites to the blackhole list, and will they remove sites from the blackhole list once all is okay? Thus, it becomes a trade-off - is it more important to talk to spammers (and a few innocents as well), or is it more important to talk to those many other systems with spam blocks (losing those innocents who share equipment with spammers)? Obviously, this must be configurable. This is somewhat controversial advice, so consider your options for your circumstance.