It is very important that you are using the appropriate authentication methods with your email. If you are not authenticating your email properly, ESPs will assume you are spamming and will filter or just drop your email.
The common types of authentication are:
Mailgun uses all of these types of authentication. When you sign up for Mailgun, we provide the appropriate records for you to include at your DNS registrar. We also provide a verification button you can use to make sure that your records are set up correctly.