Autofiltering in Pegasus Mail. I offered this for the benefit of other users who might come across this problem, like I did. Unfortunately it doesn't seem to be of any benefit to you.

I did re-associate the folder without effect. The folder is small, just 108 mails, << 2GB.
I already tried before as you describe. I cannot delete or rename the "Sent mail" folder. The server, the providers, blocks my attempts. It is weird Pmail cannot write a copy to self to the "Sent mail" folder while the mail app on my phone can and Thunderbird as well.

I did re-associate the folder without effect. The folder is small, just 108 mails, << 2GB.
I already tried before as you describe. I cannot delete or rename the "Sent mail" folder. The server, the providers, blocks my attempts. It is weird Pmail cannot write a copy to self to the "Sent mail" folder while the mail app on my phone can and Thunderbird as well.

Ahhh.... You are IMAPing to an email host. Consequently, none of my responses apply.
I have very limited experience with IMAP so have nothing to contribute.