I suggest that the Sophos Email Appliance be able to support the Microsoft limits for email.
Currently the SMTP settings on the Sophos email appliance do not support the Microsoft limits. Example: The Maximum number of recipients on the Sophos email appliance only supports 1000 recipients while Microsoft supports 10,000. Can we upgrade the Sophos Email Appliance to support these settings: https://technet.microsoft.com/en-us/library/exchange-online-limits.aspx
