Diane, have you been hearing more about this issue, especially given how plugged in you are on all things OL? Test1, how has it been going?
I just so happen to have been investigating the use of New Outlook with IMAP in order to help someone out, and in the first hour of testing, I hit this issue, though on a smaller scale.
I used my GMX account as a test and setup IMAP in Thunderbird and New OL. I sent several messages in from places like Gmail, Outlook.com, and corp M365. I didn't even know about this issue, when I noticed that new incoming mail stopped showing up in New Outlook about a half hour in. They were right there in GMX web and Thunderbird, so it was puzzling. Closing the app didn't help. Outgoing mail also didn't arrive! I soon found:
Hello - I am finding New Outlook extremely slow when syncing a couple of IMAP accounts, @yahoo.co.uk and .com. It can take hours, even days, although it does...
techcommunity.microsoft.com
The second article does make a couple interesting points, especially (which builds on what you said, Diane):
"Server security can see the traffic differently. Logins arrive from Microsoft data center addresses, not from your office. Mail servers commonly limit simultaneous connections and block addresses after repeated failed logins. Those protections exist to stop attacks, but they can also slow or interrupt a cloud service that syncs many mailboxes from shared addresses."
Whatever the reason (note that I didn't have any visible failed connections), about 10-15 minutes too late, the messages just appeared (both ends), as if by magic. Whatever the problem, there was no hint of it in the headers, nor the timestamps, and mail since then (albeit, only about an hour) has been normal again.
But this has turned me off New Outlook for IMAP. Microsoft standing in as a proxy is the unique thing about this and is likely the cause one way or another.
