Mailserver certificate contains wrong host for secondary domain

Hello all,
thanks for any help in advance.

My YunoHost server

Hardware: Some VM on a server I have
YunoHost version: 4.0.8.2
I have access to my server : Through SSH | through the webadmin
Are you in a special context or did you perform some particular tweaking on your YunoHost instance ? : no
If yes, please explain:

Description of my issue

When I setup an email account on thunderbird I get warnings about the certificate for the mail server. It seems that the cert. servered by the server is only valid for the main domain, but not for the secondary added domain(s).

This means I would have to set the MX records of all domains to the main domain.

Say I have setup example.com on my Yunohost, I can send mails and receive mail, mails server checks work well too.
Then I add example.org for a friend, he can send mails and receive mails, but the MX record for example.org still has to be example.com, otherwise mails severs connecting to transfer emails will get faults for invalid SSL.

Is this wrong? Did I do something to cause the cert for each domain not being used by the mailserver?

There is a test to test any mailserver which I used to confirm the SSL problem.

Help me to understand this problem, I would expect this can be solved, but maybe it is unusual to have several domains in use.
Or the MX entry of each following domain has to be set to the main domain?

Edit:
When I set the secondary domains, so all after the main domain, MX record to the main domain it works fine, the SSL works as expected. But with the MX set to the main domain I get then warnings in the diagnose function of Yunohost.

Edit 2:
Please ask some questions too, that sometimes helps to find the mistake in thoughts.