Error -19761 when sending mail (The send or update operation could not be performed because the change key passed...)

I’m having a problem with one user in our system. Details are as follows:

Platform: OS X 10.10.5

Outlook version: Outlook for Mac 2016 (15.26.1)

Problem: Randomly (about 5-10 times through the work day), when the user sends an email, they get an error in Outlook for Mac, the message is not sent, and the message is put in their Drafts mailbox. The error that is displays is as follows:

The send or update operation could not be performed because the change key passed in the request does not match the current change key for the item., Cannot submit message.SaveStatus: IrresolvableConflict

PropertyConflicts: Mail could not be sent. The message has been moved to your Drafts folder.

Account name: "XXX"

Error: -19761

They then open the message from their Drafts folder and attempt to resend it. It then sends without issue.

Some additional notes:

• The error started happening 2 days ago—no changes were made to the user’s machine prior to the error appearing.
• The error happens when sending to both internal and external contacts.
• The error only happens when using Outlook for Mac desktop client (the issue does not happen when sending mail via OWA or iPhone, etc.)
• We have updated Outlook for Mac to the latest available version (15.26.1).
• We have deleted/re-added the account in Outlook.
• We have uninstalled/reinstalled the Office 2016 package on the computer.

Any advice would be appreciated.

Thanks,

Kristin.