[PostFix, Ubuntu] Need postfix acting as secure SMTP server

hi, all

I'm a linux newbie and a have basic simple question.

Linode is hosting multiple domains: domain1.com, domain2.com

I don't want to receive, store, sort, etc all incoming e-mail. So I set forwarding of all incoming mails (to gmail) by using this main.cf:

virtual_alias_domains = example1.com example2.com
virtual_alias_maps = hash:/etc/postfix/virtual

while etc/postfix/virtual is

@example1.com myusername@gmail.com
@example2.com myusername@gmail.com

It works (i hope)

How do I set postfix to be an SMTP server (let's say it will be example1.com for external e-mail clients) with secure authorization and data transmission?

Thanks in advance