When you use a Digital Ocean server, you may see errors in the 'mail.log' because your shared IP address is blocked.
The easier solution is to use SparkPost, which can send emails for you.
See https://www.sparkpost.com/docs/integrations/postfix/
smtp_sasl_auth_enable = yes smtp_sasl_password_maps = static:SMTP_Injection: relayhost = [smtp.sparkpostmail.com]:587 smtp_sasl_security_options = noanonymous smtp_tls_security_level = encrypt header_size_limit = 4096000