[SM-imapproxy] listen_port=server_port: 993 works, 143 doesn't

[Grant <ema...@gm...>]

When I run courier-imapd-ssl on port 993, I can set imapproxy's
listen_port and server_port to 993.  When I run courier-imapd on port
143, I can't set imapproxy's listen_port and server_port to 143.  I
must set listen_port to another port or I get:

main(): bind() failed: Address already in use

Why does this work with port 993 but not 143?

- Grant