Question : Changing Squirrelmail's SMTP Server

Where do I configure Squirrelmail's SMTP server ? Can it be a remote server ?

Answer : Changing Squirrelmail's SMTP Server

yes it can be a remote server.
to configure it go to Squirrelmail directory then /config
edit the config.php and locate "$smtpServerAddress      = 'localhost';" directive and change localhost with the desired smtp address.

you can execute conf.pl and select server settings in it you can specify both IMAP and SMTP servers to automatically set related directives in config.php