Many ISPs are blocking what is called "Port 25" which is the port used to send e-mail. They are doing this to cut down on the amount of spam that is sent from their networks.
Nearly all e-mail sent via the Internet is routed through the port 25, the channel used for communication between an e-mail client and an e-mail server. Even though port 25 blocking will probably become an industry standard, however, the filter can create problems for e-mail servers and block legitimate e-mail as well as spam.
Port 25 blocking allows ISPs to block spam sent out through their networks, but it tends to punish the innocent that have a need to send through e-mail servers other than those belonging to their ISP. The ISPs that block port 25 require their SMTP server to be used instead of the remote SMTP server or a SMTP server running on your computer.
* Customers should not use ISP's mail servers for outgoing SMTP. Doing so will bypass Smarsh archiving, making the client non-compliant with archiving regulations.
To resolve this issue, users can do any of the following:
a) Contact their ISP to request they allow connections over port 25. Many ISPs will do this upon request.
b) Contact Smarsh to request alternative port settings (ex. Secure SMTP).
c) Use the Web Mail client (ex. Outlook Web Access) to send emails.
Some of the major ISP's that block Port 25: