One or more users are unable to submit false negative emails using the 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.
For more information about these type of EWS errors, see EWS Response Codes.
If the false negative emails cannot be submitted using the Outlook email submission add-in, these emails would need to be reported manually.