Oh, excuse me.. so, this is the situation:
- WG sits on a VLAN, Exchange sits another VLAN. Between WG and Exchange there is a Cisco router, and there isn't any blocking rules in the Cisco. When I tried to set up an external SMTP server as a mail server, and I added full access to WG to reach external network, the error message was the same. First, I thought that the Cisco between the GW and Exchange blocks some protocolls, but when I tried an external mail server (and it didn't work too), I run out of ideas...