First, you may want to check if your ISP is blocking outgoing SMTP
port by use DOS/Unix command:
c:\> telnet relay.dnsexit.com 25
If the port is open, you should see some messages like
Connected to relay.dnsexit.com (188.8.131.52).
Escape character is '^]'.
220 box7.dnsexit.com ESMTP Sendmail 8.13.8/8.13.8; Sun, 25 Mar 2007 22:31:30
If there is no response at port 25, you ISP must have blocked the port. To
go around port 25 blocking that is common practice by a lot of
ISPs, we have other ports open for your mail relay. You can try port 26,
940, 8001, 2525, 80 by using the same telnet command by telnet to
the alternative port instead of 25. If the connection gets responses from the alternative SMTP port, you should use that port instead of 25 when setting up Smart
Hosting. Here are the settings using in Exchange 2007