I've been using the Mail app in Mac OS X for a good four or five years now. I thought I pretty much knew most of how to use it, but today I've been pleasantly surprised. I've just stumbled across a nifty little feature that I hadn't known about before, and it's going to make a small, but significant improvement to the way I do email in Mail app.
In replying to an email in Mail.app, when you hit the reply button all the text from that email is inserted into the reply message (like all other email clients). However, having just been sifting through a rather complex message & trying to decipher a few key areas, I selected a specific portion of text to copy and insert into the reply message to illustrate and help clarify a few points.
This is a practice I usually follow, but in haste, I selected the text I wanted to copy before I'd created the reply message. When I hit the reply button, I suddenly realized that I had a reply message with only the selected text from the previous mail present.
If you select text in Mail.app and then hit the reply button, Mail.app will automatically insert the selected text into the reply message whilst leaving everything else behind. This is useful if you want to quote a specific piece of text in a reply message, but not make the recipient trawl through the previous conversations to get there (and it saves you cutting and pasting the text, too).
I'm sure this is quite obvious to veteran Mac users, but that's why this is a Mac 101; for new Mac users, and those slightly less proficient with email, it's a little tip to make your email experience that much more pleasant.
Let us know if you have any helpful tips for using Mail.app in the comments below.