As you have probably experienced yourself, we are living in the middle of a spam epidemic.

Today many ISPs are resorting to blocking e-mail from mail servers that have been reported as harbouring spammers. To do this, the ISPs subscribe to an increasingly large number of what are known as DNS blacklisting services. These people trawl the web and analyse e-mail traffic to find the servers that the spammers are sending their bulk e-mails from and list them by their IP numbers.

Now, the spammers don't like getting caught - so they try and relay their spam via other servers (hiding behind their IP numbers) to avoid detection.

Sadly many SMEs who have their own internal mail servers have had theirs set-up by less than qualified people, who have left their servers open to abuse. These servers are known as "open relay servers" and they are also black listed.

 If your emails start bouncing back you might want to check if you are on a DNS blacklist by visiting: http://relays.osirusoft.com/.