Error occurred while sending mail SO_OBJECT_SEND, return code &1.

What causes this issue?

An error occurred while a document designed as mail output was
transferred to SAPoffice (function SO_OBJECT_SEND).

System Response

Mail processing was terminated.

How to fix this error?

The return code refers to the cause of the error in SO_OBJECT_SEND.
Check the parameters of the SAPscript call from within the application
(MAIL sender, MAIL recipient, application object ID) and the
configuration of mail processing within SAPoffice.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.