After doing all the steps above in my last post, client was trouble-free for 6 days. Then a new "failed to update headers" error appeared.
All users for this domain are on Outlook 2010, because they HATE the new Office interface (the one starting with 2013). Since 2010 will continue receiving security patches until October 2020 when it officially goes EOL, they will stick with 2010 for a while longer.
Only 1 user is getting these errors, and she is the only one using EAS to sync the acct with her mobile device (IMAP in Outlook). Everyone else is using IMAP for both Outlook and Mobile, because they don't need calendar/contact sync.
Is there a known conflict between IMAP and EAS that would explain why only this specific user is affected?
Or is that just coincidental?