Check the configuration in ECC with t-code: SOST & SCOT
(You can ask your BASIS folk to do so).
Refer below links:
SCOT - SAPconnect Administration - Application Server Infrastructure - SCN Wiki
Manually push an email from SAP Outbox using SOST
I recall, the same thing was occurred in our case but the system restart (ECC & PI both) helped us.
I am not sure but you can try this in your case too.
Regards,
Dipen.