The smallest instance running 24/7 is $175.20 for 365 days. Add in S3 storage for 25G of data, and that adds $28.50.

This does not include incoming bandwidth, bandwidth used by your IMAP or webmail client, or DNS lookups that you'll do for every message received. (Also, spam filtering is CPU-intensive, so if you get a lot of email, a Micro instance may not be big enough.)

This doesn't include a secondary MX, either. (I like the DynDNS secondary MX service, personally, which is ~$30 a year IIRC.)

The info I get is that the smallest instance is a micro instance at just $23 for 365 days, not on demand, but a reserved instance.

Why use EC2 for this? You don't need the elasticity, and you can get cycles a lot cheaper elsewhere.

Virtualized dedicated server with SSH root for $23 a year?

Just tell me where to sign up (next year when AWS free tier expires).