I've been going through the forum for hours, it seems that everybody has a slightly different problems. Mailservers are hard.

Anyway, quick overview of the problem, hopefully somebody can point me in the right direction.

1) I have installed postfix & dovecot, and thunderbird on my local pc
2) I can send emails to any domain using my mailserver.
3) I can send an email to my domain, using my mailserver, and download it from the mailserver.
4) Outside domains however can not send emails to my domain;
they never arrive.

This looks like a dns issue

http://www.mxtoolbox.com/SuperTool.aspx
domain sooser.com returns
Hostname mail.sooser.com
Ip Address: 50.116.51.204

When I click the SMTP test, it says
No connection could be made because the target machine actively refused it 50.116.51.204:25

This is on Ubuntu 10.04
In the linode DNS manager, there is 1 MX record; mail.sooser.com
There is also an A record for email: mail 50.116.51.204

And the Current Reverse DNS is set to
50.116.51.204 sooser.com

So, which part in the DNS did I miss?
I read something on the internet about a PTR record, but I can't find that anywhere.

Thanks for all your help!

#telnet mail.sooser.com:25 resolves to your IP, but gets dropped

So DNS looks ok - check your firewall rules

Thanks vonskippy!

It had indeed stopped listening on port 25.
The one thing I didn't check

Working like a charm now