I previously got a successful hint about how to set up my Optus e-mail on my Samsung S8+ as an IMAP account. That sorted out the problem with e-mails being deleted on the server not being deleted on the phone.
Now I also need help to ge the opposite working: If I delete a mail on the phone, it does not get deleted from the server.
What setting do I have to amend to get this to work?
I think I have asked a half a dozen times what the story is with IMAP and unfortunately I have never had a response that either confirms or denies what aspects of IMAP actually work if at all. Not having an Optus email account I have not been able to try it out for myself however it has been said many times that Optus do not support IMAP and there is no recent information suggesting anything has changed. It might be best to just setup a pop account if you are having problems with mail not deleting from the server. You will need to make sure that the advanced mail server setting on your email program is set to delete mail from the server after you have downloaded it otherwise you will end up with the same problem.
I had it set up as POP at first, but that was even worse: Even if I deleted an email from the server it kept coming back on the phone.
I then got advice on how to set the account up as IMAP, and that worked much better apart from the thing that I can not delete mails from the phone.
Assuming you do most of your work on your desktop and only check mail on your mobile when you are out I would setup pop mail on both of your devices. On your desktop you would set mail to be removed from the server once you do a send and receive. On your mobile you would set your email to be retained on the server. The downside to this method is you won't get any emails you have downloaded to your desktop. It's a compromise doing it this way however with no information on IMAP you may be wasting your time trying to sync your mail correctly as IMAP is supposed to do.