I think that you could do that with scripting...
I'm guessing that even if you could do what you want, it will be on no avail, because hmailserver won't send those credentials on to whichever server was intended to receive them.
Most places simply block all port 25 traffic via their wifi and wired networks.
Any guest needs to send mail they use Webmail from their ISP, or control their own mailserver and use different ports
Just 'cause I link to a page and say little else doesn't mean I am not being nice.