I have managed to sync ALL my Android devices using the system’s internal IP.
Last weekend I enabled external (internet) access and was able to set up the Exchange account’s of a no name tablet and a Huawei P20 lite and successfully sync using the external domain.
But I was not successful with my Samsung A50 (Android 9) and I can only guess why.
On all the other devices I was able to just change the server adress (external domain instead of internal IP), but the Samsung doesn’t let me do this, I have to delete the old account and set up a new one.
In the settings there is no entry for the port (443) and no setting to “accept all certificates” which wasn’t a problem internally but externally I can’t even reach the server.
As this works with the other devices (and locally installed Outlook as well), this must be a Samsung specific problem and I hope someone else has had it also, given that the phones are often used.
I just talked to a friend of mine who thinks that this might have something to do with Kopano not having access to the Let’s Encrypt certificate which is successfully installed and refreshed.
I have marked the box “Use certificate in Apache” but not the other two (Dovecot/Postfix) as I don’t use them as far as I know.
Also, the mail domain is equal to the internally used domain, the external domain is just a DynDNS domain as I don’t want to use Kopano for external mails, just for calendar and contacts.
Thank you very much!