I see. Maybe that's because mailtrain adds unsubscribe headers in the email header etc. I don't really know of any other bulk mailer software. Are you able to contact the support of turbo mailer and ask them if they support STARTLS at all?
I made a copy of my profile folder and upgraded. My Mail Accounts were visible in settings but not in the folder pane(?). Reverted as I did not have time to dive deeper into the issue.
Also, the TbSync Addon is only in beta for 78, though apparently they are working with staff to get it integrated like Lightning.
Now I understand why so much user end by not using KMail (if you are curious look at KDE forum and Reddit for KMail issue)
Basically @girish you were on the right track simply I had to go a little bit deeper
instead of looking under identities (because, of course, my Identity was well configured)
the issue is located under the sending configuration where you have to declare the mail email (not the alias even if my test is not with the alias.)