Backsideport Option
From MsgServerDocWiki
The port the MMP will use when connecting to a message store server. This option lets you run a multiplexor and a store server on the same machine, with the store server on a different port. The smtprelays option provides equivalent functionality for the SMTP Submit proxy.
Use with the IMAP proxy
The default value in this context is: 143
Use with the POP proxy
The default value in this context is: 110
