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In your experience, which type of EC2 instance is best suited for sendy? Normal? High cpu? High memory? EBS Optimized?
I run Sendy on a Micro instance which shares a company site and a blog. It's just fine.
Bulk email applications needs sufficient CPU and memory during sending. I wouldn't say one is more important than the other.
Choosing the right instance depends on the volume of emails you need to manage or send. The best way is to 'trial and error' until you get the instance you're happiest with. Of course, some common sense is needed, eg. if you have 1,000,000 emails, don't start with a micro server. :)