probably he was connect via intranet. You just do not get .. "user does not exist" when there is no mailserver listening at all @ 25.
But I see there is no problem for him anymore.
A test on postmaster works now
220 tecandweb.com
helo me
250 tecandweb.com Hello [80.242.238.134]
250 OK Sender ok
250 OK
Recipient ok
data
354 Start mail input; end with .
bla .
250 OK
I guess he is fine now.