Tebogo Tebza is right. This is a configuration on your SAP system that require read receipt or status notification.
Such option can be enabled on runtime or via below parameters.
Table SXPARMS or SCOT -> Settings -> General Parameters.
The parameters are:
| FORCE_DSN_ON_ERROR | |||
| FORCE_NO_MDN |
FORCE_NO_DSN_ON_SUCCESS
| FORCE_NO_DSN_ON_ALL |
Note 797308 (Status notifications for incoming e-mails).
If none of the parameters are in use, then the application require the notification status. This was set on runtime by the caller application.
Regards,
Diego