What are the trusted Certificate Authorities in iOS?

I'm wondering if StartCom is a trusted CA on iOS.

I'd like to setup a mail server for a TLD, and use their free certificate. It's only about $10/year cheaper than more well known authorities, so the deciding factor is if it's trusted on my iPhone.

I know it is on OS X, Windows and many linux distributions, but would like to be certain.

I think this Apple support article has the list you're looking for. It is updated as changes are made:

iOS: List of available trusted root certificates

And StartCom is on there.

Here's the trust list for iOS 7:

http://support.apple.com/en-ca/HT203065