Mails were sent from SAP but they were received or not received by the recipient. The indication was grey in SOADM instead of green and error code was XS718. The information from system was recipient valid, delivery attempted as below.
In SOADM tcode go to
Business Communication Administration -> Settings -> SMTP Connection ->
Outbound Messages -> Settings
Open Status Requests tab.
Change Request Notification to Only if Errors
Uncheck Do not request read confirmants generaly