IDoc sent as SAPmail to user &

What causes this issue?

You can find the relevant text message in the inbox of the specified
user.
If several recipients have been transferred in type E1TXTAD segments,
then only one of the recipients is logged. A complete recipient list is
available in the inbox of the user entered.

System Response

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

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.