Autoresponders are an essential part of internet marketing.
Most people – including myself – normally suggest Aweber. It’s long established and has an excellent relationship with the main email companies, so your messages are almost certain to get delivered. It also copes with very large quantities of emails if and when your business gets to giant numbers.
But all that comes at a price and when you’re starting out, costs seem to mount up quicker than income arrives.
If that applies to you, there is a free WordPress autoresponder plugin that works well and has some scaleability as well.