Re: [exim] syntactically invalid argument(s) - rejected HELO

Marc Perkel wrote:

>>
> Yes - I think that's it! So - why is an underscore a reason for
> rejection?
>
Becuse it is invalid character in hostname.
Hostname can contain only latin letters, digits, and "-" (minus).
But today many network administrators use '_' in hostname as replacement
for space.