I have 4 Macs that all access my email on both an iCloud account and an Exchange Office 365 account. Yesterday I installed spamsieve on my iMac running 10.12 (because too much mail was being filtered out with Apple’s filter). I believe the moment I rebooted Mac Mail after the install, I lost the ability to send email on the Exchange account. Every time I hit send, I get a message “Cannot send message using the server Exchange (Exchange). The specified object was not found in the store., The process failed to get the correct properties.”
I uninstalled spamsieve but it did not help. Then I discovered that all four of my Mac machines (on different networks) are all unable to send email on Exchange (they send fine on iCloud). Sending works fine from iPhone though, and from web client and from Outlook for Mac. Seems the problem is limited to Apple Mail / Exchange. I also verified it is specific to my account because I can create another account for a different email address on the same domain on Office 365 and it sends fine. Only my account on Apple Mail client wont send and only on Office 265 Exchange.
This started the same instant that spamsieve was installed, so I have to believe it is related. There must be some setting on my email account on the Office 365 server. Microsoft tech support was not helpful (they said they only support Outlook). My internal IT is without a clue on this.