Add `local_domain` as an option to the smtp configuration. This can be used to change the domain that is used to send the `EHLO` command during the SMTP handshake.
`null` is a sensible default since Symfony/Mailer will use it's own default (`127.0.0.1`) to send the mail.
Co-authored-by: Bram in 't Zandt <bram@bigspark.com>