vpopmail (vchkpw) is a collection of programs and a library to automate the creation and maintenance of virtual domain email configurations for qmail installations using either a single UID/GID or any valid UID/GID in /etc/passwd with a home directory. Features are provided in the library for other applications which need to maintain virtual domain email accounts. It supports named or IP-based domains. It works with vqadmin, qmailadmin, vqregister, sqwebmail, and courier-imap. It supports MySQL, Sybase, Oracle, LDAP, and file-based (DJB constant database) authentication. It supports SMTP authentication combined with the qmail-smtp-auth patch. It supports user quotas and roaming users (SMTP relay after POP authentication).
The Header and Envelope rewrite mini-HOWTO explains how to rewrite your from address differently in the mail header and the mail envelope. This may be necessary, for example, if your ISP requires you to send mail with one address, but you are subscribed to mailing lists with a different address, or want others to see a forwarding address instead of your ISP's address.
Sendmail Extended Queue Groups is an m4 file that can be used to add extended queue groups to Sendmail 8.12.x. Extended queue groups allow administrators to choose multipe queue groups based on the sender, recipient, or sender and recipient pair. This improves upon the default plain queue groups feature, which can only operate on recipients.
MailStripper Pro is a mail scanner that aims to remove spam and viruses from incoming mail. It recognises and decodes many obfuscation techniques used by spammers and uses several different mechanisms to recognise spam which are immune to Bayes poisoning, and has achieved real-world success of over 99.8%. It is MTA-independent and does not implant itself into your mail server. Instead, it functions as a proxy, allowing the MTA to reside on a different server.
QUICA is a Web interface to manage various aspects of email administration. Working as a courier-mta frontend, it can make a quick and easy to use email server. Tasks like creating accounts and forwards can be done easily. It manages domains separately, allowing the delegation of each domain to different people. It's a great tool for your customers if you are a service provider, but also works well for a single domain. Quotas for each account are managed, the number of email messages and hard disk space can be controlled accurately. Internationalization is provided (with some languages included) and you can add your own. The interface allows you to delegate the management to non-sysadmins.