Short answer:
Is Fedora or CentOS good enough for the job?
Yes
Longer answer:
- I want to learn to play the guitar, which brand should I chose?
- I want to start a jogging routine, is Nike better than Addidas? or should I chose Rebook?
- I want to build a nuclear reactor, what color should the bikeshed be?
Do not waste time and energy focusing on details. It really doesn't matter which distro you use. The only wrong choice is not to make one. Distros, to a large extent, are equal in term of 'power', in the sense that they're all as powerful as the sysadmin commanding them.
For the SMTP server, you're looking for
Postfix (unless you have a valid reason to use something else). Whether it's builtin or installed via your package manager, you're going to have to spend some time configure it if you want something stable and secure, you know, production level.
Further reading
I answered a
similar question on Stackexchange a while back.