Well… here I have saw other users that have installed server without some services and if I’m not mistaken I have saw comments from @Raphael saying pretty much best option will be to backup everything, reinstall server and restore backups… I guess for this could be similar, I understand this could be frustrating, even with that amount of info…
There is something I think can be done… I have never performed this anyway, if there is an possibility to get another VPS and set it up only as mail server, and depending how you handle DNS you keep sites on current server and run email on the new one.
Hope some of this could help or at least just give a clue…
EDIT: I found the topic talking about similar request, kind old from 2020, but check it 
EDIT 2: Another topic about it here