Issues with SSL and Email accounts clients

1: Somehow in outlook and on apple devices me and my clients gets a SSL from (Domain A) but that is not nice if domain B wants to connect and does see a differtent Domain. Domain B mailaccounts should see Domain B SSL certificate.

2: Bigger problem is the security warning:
the target principal name is incorrect (outlook)

I do have setup RDNS on the VPS IP (trough host) and set this up in my dns records. How to resolve this. And please if you have the solution tell me step by step

I checked but lots off these questions about email are not answered. Also on outlook app android adding email account is smooth. But outlook 2016 it is asking all server details and in the end cannot find smtp? And also on apple mail client it keeps having ssl issues…