Using a combination of SMTP server and SMTP Proxy Server
Using secondary ports
Using secondary ports you can have one server listen on two ports for SMTP. Secondary ports can be configured for both SMTP Server as well as SMTP Proxy Server. This way most of your users will connect to the primary port, but a handful can connect to the secondary port.
Using a combination
Alternatively, you can enable both SMTP and SMTP Proxy server.
SMTP Server - Configure this to listen on port other than 25, for example, 2500
SMTP Proxy Server - This accepts emails on port 25 for regular users.
When you configure the SMTP Server to listen on port 2500, users who cannot send email otherwise will be able to connect since the ISP won't block this port.
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