Overview
One of the most important parts of an email marketing campaign is actually acquiring the contacts to send the mail to and getting them into your database. In order to keep your database up to date you should allow your contacts to update their information otherwise you will find that your contact list rapidly becomes outdated. The emails you sent out by you or your email marketing software will be undeliverable and will bounce.
Email Bouncing
An email bounce occurs when an email is sent to a particular address but the receiving mail server has sent it back with a delivery failure notification. Depending on the reason for the delivery failure the bounce can be classified as either hard or soft bounces.
Hard Bounce
Hard bounces are when emails are sent to remote servers and they become permanently undeliverable. The way emails are delivered is facilitated by the simple mail transfer protocol which communicates with the remoter server and controls the way outbound mail behaves. This deals with the destination of the mail and how it is to be delivered. During SMTP delivery the server may return an error code such as 550, signifying the email has been rejected. Being blocked by a recipients server or typing in an incorrect or invalid email address could also be causes for hard bouncing.
Soft Bounces
Soft bounces occur when an email is sent to an active address but is bounced back before being delivered. The most common reason for this happening is the mail server is down or that your recipient has too much mail in their inbox, it is only a temporary action and once the problem has been resolved you should be able to sent you mail again. Another case of a soft bounce occurring would be if the recipient is away and they have left an out of office reply you should keep these contacts as it is quite likly that they will read their mail when they return.
This is why it is important to keep you address book up to date as you do not want to be sending out mail to people who are not receiving it as it can cost you money and waste your time. Always remove hard bounces after every email campaign and carefully monitor soft bounces. Soft bounces can be kept unless you are noticing that a particular address continues to bounce over a long stretch of time. A double opt-in is also a good idea which allows clients to opt to read your mail making sure that people you send ,ail to actually want to read. One more vital action you must carry out before you carry on with your email marketing campaign is to make sure you have validated your contact list.

Author's Bio: 

The author of this article is the creator of many an email marketing campaign and understands the need to keep your contact list clear.