Pop3 not working.

This is a security mechanism, to solve spam problems. NO ONE in the WHOLE network can use this server to send out emails (besides using Outlook in Exchange mode, because Outlook has some spam sending protection).
If you really want to use the server as an SMTP relay, you risk that anyone in the network can start sending spam (via virus/trojan/etc).
Of course, if you are aware of these risks, follow this guide: http://www.petenetlive.com/KB/Article/0000542.htm#e2010
Please be mindful which IP's you're going to allow access to (keep it to a minimum!).
After that, you'll get more problems if you also want to use this for external access (now you're going to add external IP's to allow SMTP relay?). You'll set up your server up to be a spam server. Please find a way to use Exchange as it should be used (Outlook/OWA/ActiveSync, NOT SMTP!)

Then what is the way, i will able to send email securely.

You have Exchange server. Then use Outlook properly (as in add an Exchange profile to Outlook, instead of POP3/SMTP).

Yes i understood. But some of the users are roaming outside the country,  they are using outlook instead to go online check the emails.

Exchange can face the internet with only port 80 or 443. It's enough to enable users to start up Outlook as usual. Install the certificate as trusted, and fill in the necessary Outlook Anywhere info.
Follow this guide: https://support.office.com/en-us/article/Use-Outlook-Anywhere-to-connect-to-your-Exchange-server-without-VPN-ae01c0d6-8587-4055-9bc9-bbd5ca15e817

I have a single-domain cert for mail.mydomain.com. That's why i can not configure outlook anywhere.