Each software category below lists some libraries or modules as examples. Of course other software is eligible too. Many of the examples are prominent, some are intentionally not. If you are wondering whether a package would be eligible for a bounty, you can send us an inquiry about it.
The client names both Thunderbird, which is widely used, and Alpine, which is only widely used among the fourteen people who still read their mail in a terminal window. Both are common in some way, so they’re eligible for this program.
Alpine (needs C-client support first)
Dovecot (may already be in progress)
procmail (to support UTF-8 regular expressions and mime-encoded headers)
PHP’s standard library, including the C-Client
Ruby’s builtin mail modules, such as Net::IMAP and others.
PHP mail functions
Pear Mail, Horde, Net_IMAP,
WordPress form and mail plugins?
Drupal form and mail plugins?
Django form and mail modules?