Which is the best mass email autoresponder?
Bulkresponse, Aweber, Getresponse are all good autoresponders.
Aweber for small budget marketers and Getresponse for big budget marketers.
If you are looking for free autoresponder then go take a look at Mailchimp. Its also a good one.
The first & best option is mailchimp. Otherwise you can try sendgrid also.