Using TurboSMTP server for sending your emails in ERPNext is the best idea, as TurboSMTP server has about 99% inbox delivery.
Create account with TurboSMTP if you do not have it: Register now
Once your account is ready, put your TurboSMTP credentials in the appropriate fields:
Enter the TurboSMTP information:
|My server requires authentication
|your User name from the SMTP Configuration settings on TurboSMTP
|your password from the SMTP Configuration settings on TurboSMTP
Now save this information and you have successfully configured the TurboSMTP Email in ERPNext.