Some ISP's have a policy of blocking port 25 which is used for SMTP, they do this to try to stop you using an SMTP server that is not theirs.
You can perform this test to find out if your IPS is blocking port 25:
- In Windows go to the Start Menu then select 'Run...'.
- Type: telnet domain.com 25 (where domain.com is replaced with your domain name)
You should see an ms-dos window popup with an Exim welcome message (the software we use for email on servers). If there is no message it is possible that your ISP blocks port 25.
If your ISP does block port 25 you can bypass this by using port 26 instead, you will need to change the port for SMTP in your email client (i.e. Outlook). Please consult the documentation for your email client to do this.