How to block a port connection (Ex: GMAIL) for incoming mails and allow only for outgoing mails? Is there a way to block connections for incoming requests for GMAIL at port level? Is it that all protocols use same port for incoming and outgoing connections?

Block incoming ports 25, 465 and 587. And you will have only outgoing mail from this server. Also you can consider to set the SMTP daemon to listen only to 127.0.0.1 which will make it accept incoming mail only via localhost and send mail to the world