One or more users are unable to submit false negative emails using the Outlook email submission add-in.
Outlook Email Submission add-in
The Outlook submission add-in sends a request to Exchange Web Services (EWS) to get the base64 encoded email content. However, if the EWS response is greater than 1 Megabyte, the email content would not be retrieved due to the size limitation.
The size limit applies to Exchange 2013, 2016 on-prem solutions. There is no limit when submitting on a hybrid environment with Office 365, standalone Office 365 or Exchange Online.