We have an on-premise environment that is configured with Exchange Online for server side sync. The following error occurs randomly when we try to send out emails with large attachments (5-7MB in total). Once an email fails, it gets stuck in Draft folder (in the mailbox with attachments) and CRM email will remain "pending send". Then CRM tries to resend again, creating a duplicate email in the Draft folder. The duplicates will be created (every 5-10 minutes) in the draft folder until the CRM email is removed.

>Error sending outgoing mail item xxxxxxx-xxxx-xxxx-xxxx-xxxxxxx. Response from Server: <ResponseMessageType ResponseClass="Error"><MessageText xmlns="schemas.microsoft.com/.../messages">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

Thanks in Advance!