Having a clean email list assures the successful delivery of your email campaign. You must have obtained your email list from many sources. An address can become invalid or it may be your customer has opted-out/unsubscribed from you. A wrong email address can also be due to an incorrect entry or an old system that has out-dated email addresses. Having invalid addresses spoils your reputation as a sender. Therefore, it is ideal to scrub your email lists of 'potential troublemakers' periodically. To maintain an effective list it is recommended that you clean your email list at least every quarter. Here are a few questions you can ask to find out how often your email list needs a thorough scrubbing:

Where did you obtain your original email addresses from? This could be from subscribers who have opted-in, or someone who has passed on your information, or someone who visited your website or from an older database. If you have followed a good opt-in process and have verified the addresses before saving it to your database, you are assured of their validity and can reduce the frequency of email scrubbing.

How often do you send out emails? If you send out an email every month, having a quarterly scrub is more than enough to maintain a clean list. But if you send in emails only bi-monthly or once in three months or more, a list scrubbing is necessary to rule out invalid addresses each time you send your emails.

How often do you get bounces? If you are encountering more bounces than before you should be scrubbing your list more frequently. Sometimes you may find more invalid addresses in a random scrub. If so, it is highly recommended you reduce the intervals between your email scrubbing.

Each marketer's situation is unique. The answers to the above questions will aid you in deciding the frequency with which you should be performing an email list scrub. At Delivra, we can help you achieve the deliverability you desire in your email campaigns. With state-of-the-art technology our email marketing consultants allow you to maintain a clean email list and arrive at the right email solution for your company.

Sources: 

http://www.emailmovers.com/email-data/email-data-cleanse

http://blog.strikeiron.com/bid/51318/Email-Scrubbing-How-often-should-I-scrub-my-list