[RKDinOKC]

No errors setting up the account?
Does it sync if you turn off wi-fi? Make sure cellular data is on for mail. If it syncs it's a local dns/firewall issue.

Is it 365 or exchange on-line. Is active sync turned on for that account?

Microsoft 365 moves accounts around between servers all the time. Could just be on a poorly responding server. Have had that happen with a couple of users. One their new emails only made it to their desktop once a day at 3pm. 3rd day his account was moved and it stopped doing it.

All my users have mail set to Push so iPhone doesn't actually check for mail. The only thing new mail shows up faster on is if I have the OWA portal open in my web browser.

Moving to Exchange Online/365 really increased my workload and bolstered my job security. The down side is that we have had more malware/virus outbreaks, and down time in 3 months than we have had for 10 years of our on-site Exchange server.