- `HELO_HOSTNAME`: **\<empty\>**: Custom hostname for HELO operation.
- `HOST`: **\<empty\>**: SMTP mail host address and port (example: smtp.gitea.io:587).
- Using opportunistic TLS via STARTTLS on port 587 is recommended per RFC 6409.
- As per RFC 8314, if supported, Implicit TLS/SMTPS on port 465 is recommended, otherwise opportunistic TLS via STARTTLS on port 587 should be used.
- `IS_TLS_ENABLED` : **false** : Forcibly use TLS to connect even if not on a default SMTPS port.
- Note, if the port ends with `465` SMTPS/SMTP over TLS will be used despite this setting.
- Note, if the port ends with `465`Implicit TLS/SMTPS/SMTP over TLS will be used despite this setting.
- Otherwise if `IS_TLS_ENABLED=false` and the server supports `STARTTLS` this will be used. Thus if `STARTTLS` is preferred you should set `IS_TLS_ENABLED=false`.
- `FROM`: **\<empty\>**: Mail from address, RFC 5322. This can be just an email address, or