Opened client program for sending e-mails

What causes this issue?

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

System Response

The system has passed the e-mail to the client PC. If a suitable program
is installed on the client PC, the program is opened.

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.