Multiple domains and nextcloud access


I have 3 domains hosted on my mailinabox.
Email all working fine.

Let’s call my domains: domainA domainB domainC, where domainA was the first one and the master domain.

domainA/mail works AS WELL as domainBC/mail for logging into webmail.
This is nice for not confusing users of the other domain.

However IMAP requires use of domainA for all 3. (not ideal but workable)
Is that fixable with dns tweaks?

What is really annoying is nextcloud.

domainA/cloud works but domainB|C/cloud does not work.
this means I cant log into both at the same time and also confuses users.

Is there a way to get this to work?

Are you using user@domain.tld format for logging into Nextcloud?