I finally have found two workarounds that work for me.
I assume that you have the same setup as me:
- IMAP Accounts
- Attachments from SENT mails cannot be removed
Workaround #1
- Go to another computer. I could successully remove all attachments from the sent folder on my laptop (that I sent from my desktop) and vice versa: If I sent the message from the laptop, I could remove the attachment from the sent folder on my desktop.
Workaround #2
- Move the offending message from one IMAP account to another one. As soon as the "new" mail has been downloaded into Mail, the remove attachment menu item is enabled.
- It is crucial that it's another IMAP account and not only another folder within the same IMAP account. If you move it within a folder only, it may work or may not.
I think this is an issue if the mail does not leave the computer. As soon as the mail is processed in some way (e.g. on a mailserver), the issue disappears.
Hope this helps,
Regards,
Michael