Samsung Galaxy Note 10.1 — Note 5 IMAP email message "Server Unavailable" after working fine for months

I have a Verizon Samsung Galaxy Note 5 which I purchased a week before the release date. I configured two email accounts on the device using the default Samsung application. The primary is a Microsoft Exchange Office 365 account which handles email, contact and calendar. The second is an IMAP account for email only.

All has worked well since mid-August (3 months).

A few days ago, the IMAP account stopped automatically syncing. The emails drop, but only the headers. When I open an email from the IMAP account, a message is displayed, "Server Unavailable." I am unable to load the detail.

The ActiveSync account works fine - only the IMAP account is broken.

I've tried everything - remove the accounts, re-adding, etc. I've even factory defaulted the phone and started all over. The IMAP account still says "Server Unavailable."

I ended up downloading another email client, CloudMagic, and it handles the IMAP account perfectly.

It's not that I'm unhappy with CloudMagic; I'd just like to have ONE email client for both the IMAP and ActiveSync accounts.

Any ideas how to fix this issue?