Microsoft Outlook 365 refuse to setup on some Cellphone devices

Question

Good day, everyone. I trust this day finds you well.

I do apologize, as I am not a native English speaker. Outlook 365 used to work fine. I have my

own Website Design Company with my own mail server. I never used to have any problems setting up

Outlook on Android or iPhone. Earlier in the year, there was an update. That is where the problems started.

Sometimes refuse to set up a domain-specific email. If you remove an email, it refuses to set it up again (may just be a cache issue). I myself never had this problem, but I am running an older version of Outlook. I did experience this on my work phone's Outlook 365. I am not the biggest fan of the new Outlook. What sometimes does work is when you set up a Microsoft email first, then set up a domain-specific email. The problem with this is that most clients just want their domain-specific email.

This is starting to be a problem. There are different software programs they can use, but they are used to Outlook, and they understand it. Is there any solution for this?

Not sure if this is the right place, but my client has an iPhone 14. that had an email that looked like a domain-specific email, but it was not. It was a Gmail account just changed to look like a domain-specific email. We moved his emails over to our server, which is a proper Domain-specific email. The problem is that whenever he tries to load the new email, it detects the old data and wants him to log in via Gmail. I do not have an iPhone, but am I correct in assuming that after removing an email from Outlook, you have to manually remove credentials and/or clear the app cache? He is struggling to clear the cache.