Those addresses are costing you

Mark Brownlow has a post up about the hidden costs of bad email marketing. These center around brand damage, but there are other costs to poor email marketing strategies.
Previously, having old and non-responsive email addresses on a mailing list did not hurt and may have helped a reputation at an ISP. In some cases, these addresses may have even helped a reputation by increasing the number of emails delivered thus lowering the overall percentage of complaints.
More recently, some ISPs have started looking at the characteristics of recipients as part of the reputation score of a sender. If a sender is mailing a lot of abandoned email addresses, these ISPs can detect that fact. This counts against a senders reputation and may result in email ending up in the bulk folder or being blocked at the transaction.
Many senders are extremely resistant to removing old addresses from their lists. Some of the more numbers driven ones have even followed the statistics and can tell me exactly how many people ignore their email for 12 months or 18 months, and then come back and make a large purchase. This is true, sometimes people will ignore email for a long time and then come back. Keeping these people on a list may be beneficial.
However, in those recipients who ignore email (no opens, no clicks) for a long time are some addresses that have been abandoned. While these addresses are not spamtraps, repeatedly sending email to large numbers of abandoned addresses will lower the sender’s reputation over time.
All senders should have a process for dealing with non-active addresses. Allowing cruft to accumulate on a list does negatively affect reputation.