When i create a custom certificate:
1. Fil up name as required field.
2. Set "send email to students" in the options tab.
3. Set restrictions, say future date.
4. Save changes and create/design certificate.
After creating the certificate, the students received the certificates via email when in fact it's the the date yet as specified in the access restriction tab of the activity. I also tried specifying group restrictions but all students (other groups) received the same certificates via email. In moodle though, only those who passed the restrictions can accessed it. But everyone got the certificate emailed to them.