Regarding MS Windows:
It's probably important for potential email client alternatives based on pascalto be better than, or equal to Outlook Express in functionality. Even many people in the FreePascal and Delphi community still use Outlook Express, simply because Outlook Express is more powerful than many email clients out there.
Outlook express has been known to cause corrupt DBF files if you use the client agressively to sort a lot of incoming mail, so a lot of Delphi/Pascal folk are probably looking for a better solution than Outlook Express.
An email client that comes close to Outlook Express seems to be "TabMail". It doesn't have as many filters it seems, but will probably be implemented in the future. As for Newsgroups.. once again, this will need to be implemented before lots of people convert from their existing mail client.
Maybe you are looking for something other than Mozilla Thunderbird, Sylpheed claws, etc. Hopefully you can find something on this page, then.
Below are some email clients based on Pascal or Delphi.
EEmailer - Pop3 and Imap Mail Client
A free email client with IMAP, POP3 and local folder functionality. Built-in HTML 3.2 compliant webbrowser and integrated SMTP engine. Written in Delphi.
Comments: looks pretty plain and simple, in an odd GNU way.
Phoenix Email Client
Phoenix is an email client built with Delphi. It is an open source project on sourceforge.
Comments: this client probably needs more complex rule filtering. It looks like a good client, and will do well if folder based email filtering is implemented.
Cross-platform (Windows and Linux) GUI email client on source forge.
Comments: sigh, please don't try to copy or emulate outlook express.. the last thing we need is another open source project that emulates a commercial one ;-)
Sharware email client built with delphi. Older versions built with Delphi 2 and Delphi 5.. newer versions built with Delphi of some version.
Open source email client on source forge. Screenshot available!