Thunderbird is a great cross-platform basic email client.
Some questions I had to answer for myself followed by helpful links.
First, locate your Thunderbird directory.
With Debian, it's ~/.mozilla-thunderbird/default/xxxxxxxx.slt/
create a custom file user.js:
Then with your favourite text editor, add the following lines:
// Show the user agent of incoming messages user_pref("mailnews.headers.showUserAgent", true);
If you are using IMAP and procmail, you want to check other folders than just Inbox.
Add in prefs.js the following line: