""I decided to roll my own in this case because I was interested in learning about email handling after watching this great railscast from Ryan Bates. And because paying $9 per month per user for something I could probably write myself in a couple of hours seemed silly."
I find myself having to fight against this instinct almost every day. There are a couple of problems with hacking for a couple of hours to save a few bucks a month.
Firstly, you now have an additional piece of software to maintain - you're committing yourself to an unknown quantity of future work.
Secondly, your software won't get any better without you actively improving it. The nice thing about software you pay someone else for is that it gets better over time.
It's a tough instinct to fight though. Building things is Fun. It's just that there are probably other things you should be building that are more important to your company."

"As a rough guide, you might factor in another 8-10 hours, because, according to Fred Brooks, a proper programming "product" takes about x3 as long as a "program"."

killer explanation of the impact of the unknown