Outlook 2021 doesn't download email for days
My in-laws have att.net email addresses. I had to set up new PCs for them, and installed MS Office 2021 Pro (not the Office 365 version). When I added their att.net email addresses, Outlook automatically set them up for IMAP, and it worked fine. After a while, all of their folders were populated and they were getting and sending emails just fine.
However, a few days later, both of them tell me that they're no longer receiving new emails. We log into their ATT web mail, and there's plenty of new emails in their in-boxes that are not showing up in Outlook. Outlook shows no errors. It just doesn't have new emails. I restarted Outlook and it started working again, but then stopped working a few days later. Left to themselves, without restarting Outlook, it works some days and not others, for days at a time.
These two are not computer experts and would not be messing with their email settings. I don't understand how something that was working can stop working and then resume working on its own, with no errors displayed.