JSpamfilter is software that intercepts connections from remote Internet mail servers before they can connect to your mail server. The originating IP address for the inbound connection is then checked against one of the DNS-based SPAM Blackhole (DNSBL) lists maintained on the Internet by various organizations. If the check passes, the connection is allowed to proceed to your mail server; if it fails, the connection is told that the mail server is unavailable, then dropped.
dialin-spammer-block can be used to detect and block dialin clients that try to send send spam over your mailserver. A spammer is detected by watching the mailserver logfile and counting the mails sent by each dialin client. If a client exceeds the limit of mails that may be sent during a certain amount of time, the client will be blocked to relay further mails on the mailserver until the RADIUS server reports that the client has disconnected.
tMTA is a miniature Mail Transport Agent that vaguely emulates the command-line interface of Sendmail as used by many Unix programs such as mail(1). tMTA only supports local delivery to local users. It does not support SMTP or any form of inter-computer mail transport. It provides the barest functionality for daemons and users on a computer to communicate with each other using the common mail interface.
POPauthd is a Perl-based daemon that uses POP to authorize connecting hosts for SMTP relaying. It is particularly useful to authenticate roaming users with IP addresses that change often. The daemon runs in the background, watching the syslog for successful POP logins. When it sees one it enters the IP in sendmail's access database, giving the host authorization to relay through the server. The entries can be expired after a prescribed, configurable period of time with a companion script run via crontab.
The Courier IMAP/POP3 maildir creation patch adds the ability to automatically create a user's home directory when he/she is successfully authenticated for the first time. It works by calling an admin-created script that takes care of creating the directory. It is quite useful in combination with authentication-modules based on network databases like MySQL or LDAP.