Dear Pandiyaraj,
this is the normal behaviour. If you do not specify any email address in the Users Master Data (SU01),
than the sender address of this mail will be put together exactly as youdescribed: <sap-username>@<sap default domain>.
This format can be used for any other user. Both this address and the one maintained in SU01 can be used to route an incoming mail to the user's inbox.
Regards,
Attila