1. Process: Ask all the shortlisted Email appending Companies about their process. Do they have an automated or manual appending process? Automated process is best suited for large email lists whereas manual process is best suited for small lists.
2. Match Rate: Also known as append rate and refers to the number of emails that can be appended to your database per 100 records. This is a very important criterion and will give you and idea about the capabilities of the email appending companies. It generally varies from 75% to 90% among the email appending companies.
3. Accuracy Rate: Very difficult to predict and judge as a number of factors are tied in with counting the accuracy of the email. However it is still important criteria to hold the email appending companies accountable for guarantee of performance.
4. Deliverability Rate: Understand the subtle difference in accuracy and deliverability. Even if the email address is accurate it may still not be deliverable due to many factors like server level security issues etc.  Certain Email appending companies can give you an idea of their email deliverability rates.

Sales Inside Inc offers B2B Email Appending Services.