fix: format of SMTP AUTH ehlo response line

RFC 5321 (https://www.rfc-editor.org/info/rfc5321/#section-4.1.1.1)
specifies that a space must follow after the keyword in an EHLO
response.
Hence, the correct format is `250-AUTH PLAIN LOGIN`.
James currently additionally sends `250-AUTH=PLAIN LOGIN`.
This seems to be for backward compatibility with clients that
are not RFC compliant. According to the postfix docs
(https://www.postfix.org/postconf.5.html#broken_sasl_auth_clients),
this applies to very old clients, so this should not be the
default behavior.