ISP Blocking of Port 25 / SMTP Users connected to a third party ISP may have problems sending email due to the ISP blocking port 25 traffic. Many ISPs block traffic on port 25 (unless you are connecting to their mail server). If your users report problems sending email but can receive without a problem then you should check that their ISP allows traffic on Port 25 to your mail server. If port 25 traffic is blocked you can allow users to send email using port 26 by setting up a firewall rule on your server or network appliance to direct traffic coming in to port 26 (external) to port 25 (internal). Note that MailSite 9.5 and later provides support for services to listen on multiple ports. Now users can change their SMTP settings to use port 26 for the outgoing mail server within their email client. This will not prevent other users connecting to port 25 as normal.