E-rec : Mail is Not Sending after Registering in External Candidate link

Hi Dear Friends,

                             Here i am facing a problem in E-rec production . Once i filled all the details in External candidate registration page i am getting success message like your request has been created please confirm your mail id . But i son't receive any mail. But some time it is working ... In Quality it is working fine , In production only sometime it is working some time it is not working , I checked SOST , No waiting or error status nor sent status.

If i try to register again with the same user id or email , it shows it is already existed .. I guess candidate is getting create but mail is not sending , later i checked su01 i could find the register user details.  But without email confirmation candidate can't go further .

Please help me to fix this ..

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Thank you friends.