RESOLUTION
Problem: It's impossible to send mails throught Exchange SMTP server, no connection to 25ths port from specific networks.
Explanation: Some ISPs block 25ths port on their firewall. In this case customers cannot send mails through your SMTP server.
Resolution: Add 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 "Properties" on "Default SMTP Virtual Server". In "General" tab in "IP address" section press "Advanced" button. In "Advanced" window press "Add" button. Leave "(All Unassigned)" in "IP address" field and enter alternative port (for example 2525) to "TCP port" field. Press "OK" three times.
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