When you send an email, it starts out on a server. If you use an Email Service Provider, it sends your email to Internet Service Providers such as AOL or Yahoo! and email programs such as Outlook. These programs and service providers try to sort the email and send only the good email to their customers. When something looks unwanted, they send it to a junk folder or return to the sender. The process is pictured in Figure 11.1.

Figure 11.1 Email delivery is a complex process.

Nonexistent addresses are addresses that do not exist or have been cancelled. The address may contain a typo, or the individual associated with that address might have left the company. For example, a simple typo like “[email protected]” instead of “ xxx@constant contact.com” would prove to be an email address that doesn’t exist. In addition to actively cancelled email addresses, many free services cancel addresses that have not been used in as short a time span as three months.

Undeliverable email addresses are email addresses that cannot be delivered because the receiving server is unavailable, overloaded, or could not be found. Therefore, you may find that there are groups of emails with the same domain (the part after “@” in an email address) in this category. Sometimes these problems correct themselves and the emails are successfully sent in subsequent mailings.

Mailbox full indicates that the email was not accepted because the recipient’s email box was over the storage limit. You will not be able to successfully deliver email until the contact frees up some space in the account.

Vacation reply or automatic response emails include bounces that were due to an automated vacation message. In this case, the email was delivered. The email recipient has gone on vacation and has set up an automatic “I’m on vacation” email.

Block-lists contain lists of domains or IP addresses of known and suspected spammers. If a sender is on the list, its emails likely will be blocked. It is common for an Internet Service Provider to use a block-list to determine which emails should be blocked. Unfortunately, these block-lists also contain many legitimate email service providers and other senders. Just a few spam complaints can land an IP address on a block-list despite the fact that the ratio of complaints to volume of email sent is extremely low.

SpamCop

This block-list adds IP addresses to its list based on the ratio of spam complaints to volume of email sent. An IP address can be added and removed several times even during a 24-hour period depending on the frequency of sampling by SpamCop.

SpamHaus

This is a popular and free block-list used by ISPs and corporate networks. SpamHaus also runs ROKSO (Register of Known Spam Operations) that lists spammers who have been thrown off of ISPs three or more times.

Another method for sorting email is called filtering. ISPs filter emails based on their content. Filtered email isn’t returned to the sender. Instead, it’s moved to a folder other than the inbox. If there are “spammish” terms or phrases used in the “from,” “subject,” or “body” of the email, a filtering system may sort the email and keep it from being delivered to the inbox. This usually occurs on a per email basis and is typically used by corporate networks and ISPs.

It is common for ISPs and corporate networks to create their own custom set of criteria for blocking, bouncing, and filtering. Many ISPs use information from block-lists and content filters in a “weighted” system that gives “spam points” for each offensive piece of the message and then sets a threshold appropriate for their system. All email with “spam points” above the set threshold will be tagged as spam, filtered into in the trash, or bounced back to the sender.