This is somewhat similar to Email client to send unmodified HTML emails but it's 5 years old and the product in the accepted answer does not exist anymore.

I need a desktop e-mail client for sending HTML e-mails. I do not care about receiving e-mails at all. For penetration testing mail clients I need to be able to send HTML e-mails that arrive exactly like I've sent them. What I mean by that is:

  1. I want Content-Type: text/html, no multipart/*, no plain/text
  2. The client does not alter my HTML message body in any way. It doesn't parse it, serializes it back or anything along those lines.

I don't care for rendering of sent message. I'd even prefer if the client doesn't render the mail at all, just show me the raw source including headers.

Python, mail, mailx, s-nail are all a pain to use. Please don't recommend command line tools, I know they exist. And I will use them when appropriate, e.g. for fuzzing. I want the UX of a desktop client, not the command line (e.g. having a list of sent items that is persisted, not having the password in my .bash_history or code).

I've tried Thunderbird with the ThunderHTMLedit add-on. In theory this would do exactly what I want. Except that it doesn't. I've set the send options to "Send the message in HTML anyway". Once the HTML becomes non-trivial and contains things like data URIs it is converted to multipart/related and the data URIs are removed. It also adds <html>, <head> and <body> elements I omitted. I respect that Thunderbird tries to "repair" the mail so the average user runs into less issues, but I don't want it. And about:config doesn't seem to have a flag to disable the HTML parsing/fixing.

If there is a way to fix Thunderbird I take it. Otherwise is there a simple mail (or SMTP only) client that suits my needs?

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