Is it possible to receive mails from outside mail servers like gmail.com,rediffmail.com,yahoo.com etc..
Of course !!! Make sure you set the proper MX in your DNS, make sure you have your domain set-up in iRedmail and so on (the usual stuff).
Similarly, Can i send mails to outside mail servers ?
Again, of course you can, but for best results make sure you have the proper reverse DNS, SPF and DKIM entries.
Similarly what port numbers to be opened on my corporate firewall to do so ?
The usual ports...
25 for exchange SMTP
587 and 465 for submission SMTP (unencrypted & crypted)
143 and 993 for IMAP
110 and 995 for POP3
80 for HTTP