No Standards for Bounced Email Reporting
There are hundreds of different mail servers and configurations. There is no standard for reporting bad addresses and you cannot always rely on error codes to tell you exactly what is happening. In our own tests we were surprised to find that a 550 error does not always mean the subscriber address is bad.

Categorize Bounces
Bounced email should be classified into different categories. Those that are classified as definitively bad should be separated from the ones that are returned because "Mail Box is full", the person is on
"vacation", or the email is "rejected" due to filtering. In addition, there will always be bounces that come back for no clear reason. This could be a temporary server failure, a DNS problem, or an oddly worded server auto-response that gives no information at all. This would fit an "unknown" category.

Problems with SPAM filtering and block lists
Email that is returned with an "Access Denied" message or 552 error is usually returned due to SPAM filtering software. There are different reasons why this can occur. In some cases, the actual content of the message is being analyzed according to default or custom rules. Programs like Spam Assassin will pickup certain phrases in the message, like the word "FREE" or "Unsubscribe" and classify the email as SPAM. In other cases, the filtering server is referencing a SPAM "black list" and denying access on a temporary or permanent basis. For example, SpamCop will put a company on its black list if it receives complaints according to an undisclosed formula. This can be problematic when using email marketing and list services (as most small to medium sized businesses do) since SpamCop does not distinguish between complaints from one email marketing account and another. So if just one complaint (justified or not) is made against 5 or 6 separate accounts, SpamCop will count this against the mailing service IPs and will inevitably add that service to the block list for a period of time.