Message cannot be processed because Secure E-mail possible by SMTP only

What causes this issue?

The messsage that you want to encrypt or sign could not be processed in
the SAP system because it was not sent by means of the SMTP node;
security functions can only be used if you use the SMTP interface.

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?

Use transaction SCOT to make sure that the recipient of the message is
in the address area of the SMTP node.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.