User not receiving mails through CRM Production system

Hi,

If there are any errors in the workflow, the user is not notified. But sometimes the user get the mail and sometimes donu2019t.

I checked in SCOT, SOST --

Msg - 672 - Still no entry in queue

Wait for communications service

Please help & suggest. Sometimes the mail reach the user & sometimes don't (as above message).

Thanks & Regards,

Sameer