Here’s a great opinion piece from the blog of my DNS provider entitled: Want to reduce email spam to your mail server? Stop using backup spooling.
It is with regret that we have come to the following conclusion, but here it is: Offsite backup SMTP spoolers and backup mail exchangers have become worse than useless.
The problem is spam and the software that delivers it exploiting the weak authentication schemes inherent in the SMTP protocol itself. It used to be an annoyance, then it became a concern, it is now an epidemic and has resulted in the death of the offsite backup MX handler.
The author then goes on to explain what the problem is and why you won’t really miss your backup spooling. It’s a very interesting point of view (in a good way) that’s worth considering.