List poisoning is a technique used by email spammers. In this process a spammer will sign up invalid or junk email addresses to a mailing list. The signing up can be done by the spammer by using a program or by doing it manually. Since it is difficult to look at the email list frequently, list owner will probably have no idea that junk email addresses are added to his list. Eventually, the bad email addresses in the list increases significantly. Now, since most of the email list management software charge list owners on the basis of number of emails sent in a month, the expenses of the list owner increases in sending email newsletters. Worse, email senders will notice that there are many junk email addresses and many of the emails will be blocked in the email sending process. Thus, eventually a legit list owner will be blacklisted in many servers. Along with junk email addresses spammers can also add spamtrap email addresses in your list. These addresses will be checked by the email senders and the list owner may get flagged as a spammer even if he is collecting email addresses by using an opt-in procedure.
How to prevent your list from poisoning?
To prevent your email list from poisoning, make your sign up form double opt-in i.e. a confirmation message should always be sent to the subscribers’ email address when they sign up. When the subscriber clicks on a link in that confirmation message or responds to it then only the email address should be included in the list for future blasts. Using double opt-in procedure completely eliminates email list poisoning. These days almost all the top-notch email list managers have this procedure in place.