Get Response.com
This "smart" auto-responder is one of the fastest, most sophisticated, and most feature-packed, automated, customizable autoresponders on the web. (Note: The features listed below may vary between the free and paid versions of the service.)

• Instant delivery within 20-30 seconds.
• Unlimited number of automatic followups. 
• Unlimited accounts (create as many as you want.)
• Unlimited message length.
• Unlimited message changes.
• Easy set-up and customization. Just cut and paste your "ready-to-go" HTML forms to your website.  Add your own logo and advertising to the "Remove" page!
• Smart tracking (know precisely which autoresponder and message are working best.)
• Database of your prospects' email addresses and custom information with powerful mailing list management features. Automatically maintains your mailing list and keeps it clean (bad email addresses are automatically removed for you.)
• Automatically converts HTML messages into plain text for non-HTML email software.
• Automation Features — Have multiple accounts work together, seamlessly, to save heartaches or boost your sales.  If one user unsubscribes from one account, you can automate the addition or removal of that person with other accounts.