SymptomsIt is impossible to send mail through Exchange SMTP server, no connection to port 25 from specific networks.
CauseSome ISPs (Internet Service Providers) block port 25 on their firewall. In this case customers cannot send mails through your SMTP server.
ResolutionAdd additional SMTP port, for example 2525 and recommend customers to use it.
For all SMTP Front-ends perform the following actions:
Open Start > Programs > Microsoft Exchange > System Manager.
Expand Administrative Groups > First Administrative Group > Servers > SMTP Backend name here > Protocols > SMTP.
Right click on "Default SMTP Virtual Server" and choose "Properties". On the "General" tab in the "IP address" section press "Advanced" button. In the "Advanced" window press the "Add" button. Leave "(All Unassigned)" in the "IP address" field and enter alternative port (for example 2525) in the "TCP port" field. Press "OK" three times.