Do you use Apple Mail on your Mac? If so, note that 15 indie developers who create plug-ins for Apple Mail have banded together to offer their El Capitan-ready utilities at discounts ranging from 33 to 90 percent. In “Take Control of Apple Mail,” Joe Kissell says that he’d probably switch to another email client if it weren’t for Mail’s extensibility — along with AppleScript, he relies heavily on the plug-ins CargoLifter, Mail Act-On, MailTags, QuoteFix, and GPGMail. The first three of these aren’t free, but all are 50 percent off in the Mail Capitan sale through 18 October 2015. In fact, you can also save 50 percent on “Take Control of Apple Mail” itself in the sale, so this is a great opportunity to read about numerous useful Mail plug-ins and get those that will make Mail work better for you.