Advanced Perl Programming, by Sriram Srinivasan (O'Reilly, 1997) has a good section on programming network servers.
Effective Perl Programming, by Joseph Hall with Randal Schwartz (Addison Wesley, 1998) is a useful book for picking up Perl idioms.
http://www.cauce.org/ is a site from Coalition Against Unsolicited Email. There are many sites devoted to fighting Unsolicited Commercial Email; this site is a good place to start. It has pointers to many other sites, including those that go into greater detail about the analysis of mail headers for this process.
http://www.eudora.com/developers/scripting.html contains information for scripting Eudora with pointers to other AppleScript references.
http://www.microsoft.com and http://msdn.microsoft.com have information on "MAPI," "active messaging," and "CDO." The name of this technology has changed twice so far, so I'm hesitant to give you a specific pointer. There is plenty of good information at the Microsoft sites (especially in the MSDN library section) on these topics, but it tends to move around a bit.
Perl Cookbook, by Tom Christiansen and Nathan Torkington (O'Reilly, 1998), also addresses the programming of network servers.
RFC821:Simple Mail Transfer Protocol, J. Postel, 1982.
RFC822:Standard for the format of ARPA Internet text messages, D. Crocker, 1982.
RFC954:NICNAME/WHOIS, K. Harrenstien, M. Stahl, and E. Feinler, 1985.
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