MrSako, it's me again!
IfI understand you previous posts correctly, your thunderbird machine is remote from the server (ie you will want to connect from the internet.
Have a look here
at the pop-before-smtp section. What you need to be careful of is that you don't create an open relay - spammers will love you and the VPS guys will hate you.
What pop-before-smtp does is effectively require you to log on with POP before postfix will relay mail for you. This means that you have to have a valid username and password.
There may be other ways around it, but take note of the open relay point above.
I haven't tried pop-before-smtp myself though.
Edit - Also note, that you will need to adjust your firewall again to allow for pop or imap connections in this setup.
Also, you should think about using pops or imaps to avoid sending plain text passwords of the net.
Edit 2 - this is worth a read