There are a number of reasons why your outgoing emails might not work.
1) Can you use Webmail (eg. http://webmail.yourdomain.co.uk) to send/receive email?
2) If you are using any third party email software (such as Outlook, Entourage, Thunderbird, Apple Mail, etc.) and you are being blocked by the Spamhaus PBL listing when you try to send email, the reason is simply that you need to turn on "SMTP Authentication" in your email program settings.
3) Make sure you are connecting to your mail server's 'authenticated mail' port 587 and not the ordinary 'unauthenticated' port 25.
To advise further, we really need to see a copy of the error EXACTLY as it is shown by your application, this can often lead us directly to the cause of the problem you are experiencing.
If you don't have a screen shot program on your computer the best way to do it is this:
1) Hit "Print screen",
2) Go to your Start menu, All Programs, then Accessories
3) Open up your Paint program
4) Go to "Edit", scroll down and hit "Paste"
5) Click on "Save", name the file and then save it as a JPEG
Attach your screen shot(s) to your ticket so that we can better investigate the problem you are experiencing.