The quality of any email list gradually decays over time. The rate of decay will depend on many factors, from the consumers or industries you serve to the countries you do business in. 

Why does this happen?
Simply put, email accounts get abandoned and people change jobs.

In the email marketing industry, list decay is known to occur at an approx. rate of 2.1% each month which translates to almost 25% per year. Here is one of many articles on the subject:

If you have not sent to your list for more than a few months then we recommend that you consider verifying it. Even if you have verified it in the past, many of the addresses are likely to have changed status from Valid to Invalid. Don't risk getting your account suspended by your email service provider as a result of too many bounces.

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