stunnel can't bind to 0.0.0.0

Question

I keep getting this error when trying to use stunnel to work between my sbs2003 server and office 365 mail server:

stunnel: bind permission denied (wsaeacces) (10013) to 0.0.0.0:110

Does anyone have any ideas as to what may be holding onto 110 or what I can do to release it?

Solution

There are two common causes for this error on Windows:

1. A program is already listening on the port you are trying to bind.

   You can inspect the output of netstat -ab to determine if a process is already listening on port 110, and which process it is. After killing or reconfiguring that process, try binding the port again.

2. The firewall is preventing the program from binding to the port.

   In this case, you will need to configure the firewall to permit stunnel.exe to listen on the port.