I’m working on an anti-spam project (centmail) that involves having the sender use a client plugin that adds a custom header (as well as a signature, but that part’s easier). The general problem is to add an email header to your outgoing mail where the contents of the header field is determined dynamically for each message.
Surprisingly, this does not seem to be possible in Pine/Alpine. Pine lets you specify a filter script for outgoing mail but only the body and not the headers get passed through it. You can of course define custom headers in the Pine settings but I see no way to dynamically change the contents of them.
I’d be grateful to hear how to do this in other email clients (Thunderbird, Outlook, Evolution, Mutt, etc). And of course if I’m wrong about Pine.
You can do it in Eudora.
You can do it programmatically in Outlook with a custom form.
You can do it in Mozilla from about:config.