Why does VPHosting DNS Blacklist insecure servers?

http://www.ftc.gov/bcp/conline/pubs/buspubs/secureyourserver.htm

What is DNS blacklisting?

DNS blacklisting - If you are a known spammer, we use 4 reverse DNS blacklists that maintain databases of KNOWN spammers and have blocked these servers from sending mail to your account/domain. 

1. www.spamhaus.org
2. www.spamcop.net
3. www.dsbl.org (multi-hop and List)

People are saying that they get an error when they send mail to my address?

When a person using a vulnerable mail server tries to send mail to your address, they will receive the following error message. DSBL.org's definition of blocks 

(reason: 554 The IP Address of the sender (example: 24.93.47.40) was found in a DNS blacklist database and was therefore refused.) 

This means that their mail service provider allows ANYONE to send mail, and is blocked by one of the four blacklists above, as a result. This is not an error, this is what it is supposed to do to protect your mail box from vulnerable and insecure mail servers that are a haven for spammers.