The joys of email filtering

Once you start offering email filtering people expect it to work 100% of the time and are very unforgiving when they get one or two emails that they shouldn’t have.
The biggest issue facing anybody offering professional email filtering is balancing the “spam” vs. “ham” (desired email). A lot of email marketing, albeit desired by the recipient, can be easily qualified as “spam” due to badly formed mail headers, bad HTML and other elements. Choosing the threshold to block mail is always going to be a moot point.
Anybody who is involved with mail filtering will appreciate this. No matter what you do there will always be a small number of clients who are not happy with the fact that they receive 2 spam emails a day, while they used to receive 100.