I think it's iCloud
It's Mail Days to Sync, and only Outlook has this feature now, starting with 1 day, three days, ... up to No Limit.
Which email account is showing this behaviour? Gmail, Outlook, iCloud, or a different one?
Thank you.Outlook, and many corperate/business email accounts. I was trying to cover all bases by saing Exchange, but I should have known not everyone would understand that. Thanks.