SqWebMail

SqWebMail is the webmail module that's bundled with the Courier mail server. SqWebMail can also be packaged separately, and used with any other mail server that uses maildirs. NOTE: SqWebMail DOES NOT support traditional Mailbox files, only Maildirs.

Features

SqWebMail is very fast, and light. It accesses maildirs directly, and does not use an IMAP server.
Dynamically-generated HTML is stored in external files. Many (but not all) display elements can be customized without changing the program code.
Hierarchical mail folders, and shared folders.
Virtual accounts or system accounts. Virtual accounts may be managed via LDAP, MySQL, PostgresSQL, or flat files compiled into a GBDM/DB database.
Address books.

Extensive MIME support; defanged HTML mail; flowed text format; DSNs; and digests.
Javascript and cookies are not required. (There is some Javascript and cookie usage for some value-added stuff, such as print preview).
Spell checking with ispell.
Mail filtering with Maildrop.
Encryption and digital signatures with GnuPG.
Calendaring.