This is incredible. Recently for a few business customers I have taken on the challenge of HTML signatures. Instead of an attachment showing up on every email with a logo in the signature, an HTML signature references a logo on a web server, just like html-based emails reference lots of pictures in a web frame. The result is that you don't see an attachment that isn't meant to be opened, and you do see a gorgeous signature complete with branding a la your company logo.
Once the HTML code is pasted into the requisite spot, you can make some changes like increasing and decreasing font size without breaking the HTML. This is also a really cool feature I'm impressed with.
The bottom line here: I was able to implement an HTML signature using Postbox in under 5 minutes, versus the 45 minute painful process in Mail.app.
What all this means: Your customers and colleagues can see your logo in your email signature, without seeing an attachment on your message and thinking they should be looking for something to open.
Google Apps for Business (and Gmail) integration
I have one customer to methodically saved specific emails to folders (aka Labels in Google language) as part of his email processing. He upgraded to Mavericks on October 26, and instantly called me following the upgrade. Mail.app was configured with some fine-tune settings to make Google operate better. Not saving drafts locally for example, and instead saving them to the server.
The upgrade to Mavericks manifested an interesting, and unpleasant issue for this customer. He methodically processed his emails daily, saving emails into upwards of 40 folders (Google calls these labels) and deleting the rest. On October 26, his Mac's mail.app systematically removed all emails from the server (everything) and filed them away in one folder locally on the Mac. This is as close to email armageddon as one can get.
Postbox doesn't just not delete your email, it actually plays nice with Google Apps for Business. Labels are supported, and the speed is great.
Microsoft Exchange support
Microsoft Exchange email boxes can be read by using IMAP. If your Microsoft Exchange provider or IT guy hasn't set you up for IMAP support, tell them it's time. I tested this for a customer who was paying a small service provider for hosting their Microsoft Exchange server. Fortunately they had enabled IMAP support. A few extra steps are required to subscribe to folders in your Exchange mailbox, such as your sent folders and other folder, but these topics are covered beautifully in Postbox's support site here.