mail/spamd - The NetBSD Packages Collection

Tarpits like spamd are fake SMTP servers, which accept connections but don't
deliver mail. Instead, they keep the connections open and reply very slowly.
If the peer is patient enough to actually complete the SMTP dialogue (which
will take ten minutes or more), the tarpit returns a 'temporary error' code
(4xx), which indicates that the mail could not be delivered successfully and
that the sender should keep the mail in his queue and retry again later.
If the spammer does, the same procedure repeats. Until, after several attempts,
wasting both his queue space and socket handles for several days, the spammer
gives up. The resources the spamd server has to waste to do this are minimal.

If the sender is badly configured, an uncooperative recipient might actually
delay his entire queue handling for several minutes each time he connects to
the tarpit. And many spammers use badly configured open relays.