You shouldn't be using your corporate email server to send transactional email. Why? Well, for one, it will bog down your corporate email faster than you can say Exchange if you have to send more than a few emails at once. If you're running an eCommerce site, every action a buyer takes on your sites can result in transactional email. Do you really want to have a bottleneck that's so easy to fix?
Mandrill is, in my opinion, the best transactional email service available. From their WordPress plugin, which makes setup a breeze for WordPress and WooCommerce users, to their robust API that lets you integrate with anything you can send data to it with, there is absolutely no reason why you should still be relying on a corporate email server for transactional email. Oh, and if you already use MailChimp for marketing emails, you get additional transactional emails per month on a paid account!
On free plans, there is a very strict limit on timed storage of sent mail. This can make it very hard to research an issue a customer has had with a previous email. However, this is a minor price to pay when you're on a free account. If it's a deal breaker, just move to a paid account!
Just use it! There isn't a reason not to use a transactional email service!
Transactional email should not be processed by the same mail server that handles your corporate email. By using Mandrill, we are able to endure large traffic spikes on our eCommerce sites.