I can get e-mails to my phone but cannot remove them from the server when I delete them
I have 2 e-mail accounts on different servers. My business one works fine with my Samsung S8. On the Mozilla account on the optonline server, I get e-mails but when I delete them from my computer they stay on my phone and I need to do a manual delete.
Opaite Mbohovái (1)
Perhaps you set up your "optonline" e-mail account in Thunderbird as a POP account, not IMAP, and further have Thunderbird set to not delete e-mails from the server.
If the e-mail address was set up in Thunderbird as IMAP, then it would work as you expect.
Check your e-mail settings in Thunderbird, and with that e-mail provider. Read this article for how to Switch from POP to IMAP account