Jeanniey Mullen suggests to review your email efforts and ask yourself the following questions:

Opt-in environment:

• How simple is it to access your opt-in environment? Is your subscription link buried on your Web site?
• How well does the opt-in process explain all the communication types available and establish relevance?
• Does your opt-in process decrease attrition by providing examples of the content people will receive if they sign up?
• Is there a place where potential subscribers can see samples or access live versions of prior e-mail content (old newsletter versions, etc.)?
• Is the registration process labor intensive? (If e-mail isn’t one of the first four requested fields, you run the risk of losing up to 50 percent of interested parties.)

Creative and content:

• Does your subject line advise the e-mail’s purpose and the offer?
• Is
  the copy format readable under all e-mail circumstance? Does it render
  well on a mobile device or BlackBerry? Are the text versions actually
  designed?
• Is the message visually optimized? For example, are images strategically placed to move the eye toward the call to action?
• Does the e-mail’s layout support the brand? Does it carry a consistent brand message through the entire user flow?

List and usage:

• Do you have an established cadence that’s clearly communicated to the readers (e.g., a monthly newsletter or seasonal update)?
• Do
  you have a third-party list rental strategy? For example, are you
  renting lists your competitors just used? Do your offers under-perform
  compared to your competitors’?
• Do your programs have triggers
  built into them? Meaning, when someone opts in, does she receive a
  welcome e-mail? When she downloads a white paper, does she get a
  follow-up e-mail to verify receipt and take advantage of a cross- or
  up-sell opportunity?

Source: Clickz.com