Apps always log email activity and errors.
Log entries can be related with:
- Emails sucessfully sent.
- SMTP errors.
- Erros related with domain/MX DNS zones.
- Apps/users deactivated.
- Blacklist/auto-blacklist email addresses.
... and more.
These log entries are deleted by 30 days, but you can use get email logs call for saving pemanent them in your own database.