Expands on an earlier post...

Email list attrition is where subscribers leave your list by unsubscribing or by their addresses become invalid faster than new subscriptions are acquired. Addresses can become invalid for a number of reasons such as the subscriber changes jobs, or when a webmail address has been inactive for a long time. 

The attrition rates we see varies from list to list. We would expect the attrition on average to be about 15% per annum, but it can be higher. For example we'd expect to see a higher bounce rate for first time deployment to a list that had no confirm or double opt in email.

Reasons emails are junked really is a huge subject and is always changing. The three main categories in understanding deliverability follow with some key points for each:

• Infrastructure
  • The set up of your sender domain and IP address has an impact on delivery. Any issues are usually easily resolved and is part of our normal service
  • Infrastructure includes (but not limited to) SPF records and domain keys, IP isolation and whitelisting
• Content
• • Content may be filtered at several points along the path of an email - by a firewall or spam appliance, by the mail server or at the local inbox
  • Content filtering varies between ISP's, corporate mail servers and even individuals
  • Testing eliminates most content filtering issues and should never be overlooked
• Reputation
• • Black Lists (AKA block list) - if your list contains spam traps or if you exceed a threshold of complaints your ip address could get published to a black list - while usually not irreversible, if you are on a black list this is a serious concern and the cause needs to be addressed before you can be removed
  • Spam Trap - a spam trap is an email address that is part of a list of emails that have no legitimate means of becoming subscribed. Typically they are created or maintained by an isp or spam filtering service and are specifically to catch spammers. Existance of a spam trap in a list indicates either that their has been insufficient list hygiene (removal of invalid addresses) or the list was acquired via dubious means without consent of the 'subscribers' (for example by web spider) 
  • FBL complaints - Many ISPs offer a mechanism to identify an email as spam - these are aggregated by the ISP and contribute to your sender reputation. If the ratio of complaints exceeds the threshold set by the ISP they will begin deferring or blocking your email. Some ISPs offer a feedback loop from these complaints so that the sender can remove the complainant - if this service is offered it should be used.
  • ISP's evaluate subscribers behaviour towards a sender including the ratio of emails that are marked as spam, deleted without being read and whether recipients are even opening your email