Just as you regularly take out the bins, clean the office and re-order stock in your "real" world business, there are tasks that should be performed regularly to keen you online business running smoothly.
Keep Your Mailing Lists Clean
It is an unfortunate part of life on the internet, but you will get people and automated spambots signing up for your email newsletters with fake or temporary email addresses. It's important to remove these from your email recipient lists on a regular basis. Repeatedly sending emails to invalid or expired addresses will result in ISPs flagging your legitimate email marketing efforts as spam. It will also give you a false impression of how well you campaigns work when reviewing reports.
A day or to after you've sent out an email campaign you can easily remove all bounced emails from your mailing list with one click just by going to the Email Addresses That Bounced option in Reports and Performance for your campaign and clicking Unsubscribe All under the Actions tab. You should then do this just before sending your next campaign.