IMAP server synchronization

xfinity IMAP server has stopped synchronizing. Checked settings to make sure they are all the same and appropriate, but eventually gave up. I then deleted the account on all three devices I have and then added the accounts back in later. There is no evidence of any synchronization after initializing the account. For example I have to delete an email individually on all three devices. On the two iOS devices it doesn’t even know there are “read” emails, they think all the emails are unread, and I had to select all and say all read.  MacOS mail did get that part right. They’re an iPhone, iPad, and MacOS laptop, and they’re up to date. Using Apple mail on each device. IMAP is pretty simple. Any ideas on this? I know of other people to whom this is happening also. -Jim