It's not that I couldn't use courier or X-mail or vpopmail or whatever but the out of band projects aren't logical choices for someone that doesn't want to commit to maintaining their own upgrades - especially when considering that the RH packagers have provided reasonable enough alternatives.
I'd just like to say that some, if not most, of the 'out of band' solutions don't need upgrading.
The OP obviously did some research and planning and rather thoughtfully offered his conclusions for review.
I'm less than certain that the OP was well served by suggestions of out of band alternatives. Just using one of the daemons supplied in CentOS and used by a fair cross-section of users on this list provides a comfort level that most out of band packages could never deliver.
There is a reason why many large sites use 'out of band' packages.
Redhat has geared its offerings for those who don't have a lot of time on their hands which is the case for most SME's I'd say. Beyond that, the bundled packages are not up to special environments.