If you are distributing your surveys via email, its very important for you to understand the delivery rates and other email delivery statistics to improve your delivery rates and inturn your response rates. There are a lot of factors involved here, for instance your content plays a very important role with delivery rates. Certain email client may flag the content as junk and reduce your delivery rates. Now to understand all these factors its important to have access to the delivery satistics and be able to experiment to improve delivery rates.