SAP Note 23858 - E: Transaction SM53 is unknown

Symptom:

When you start the transaction to open a remote connection to SAP using the menu path 'System -> Services -> X.25 Service' the following error message appears:
E: Transaction SM53 is unknown.

Cause and prerequisites

Transaction code SM53 not delivered.

Solution
The ABAP/4 'SAPSERVR' with the transaction SA38 can be started as an alternative way of opening a remote connection to SAP.