In Claws Mail, is there any way to insert / modify headers through an external program?
According to this documentation on Actions, there does not seem to be any way to do this. Specifically, when running an action on a message that is being composed, all the ways of feeding the entire message text (rather than just the message body) to a script are barred from usage.
I ask because I would like to implement my own kludge for composing HTML mail on Claws. The developers have their own very valid reasons for not implementing HTML composition in claws. However, it would be simple to write a bash script to convert a Markdown formatted email into HTML and insert the HTML version below a multipart alternative header. If this is implemented as an action on message text, Claws can simply send the email with both parts. However, in order for this message to be read properly, Claws has to also permit my script to add in a "Content-Type: multipart/alternative" header into the headers of the message. I am not sure if this is possible.