Windows + telnet + Sendmail = 501 5.0.0 HELO requires domain address

[wettek]

I'm running Sendmail 8.13.8 on a domain-joined Linux server and am trying to send mail from a Windows host, over telnet. When I run the command, "telnet server.domain.local 25", I get back, "501 5.0.0 HELO requires domain address". If I include domain.local in the HELO command, I can send mail.

Why am I getting this error about the domain? More importantly, how do I prevent this?

Thanks.