Most of you probably use more than one email address, so when you start a new message in Mail, which account does it use? The default setting is to compose from whichever account the selected mailbox in the sidebar belongs to.
Since I've selected a Gmail mailbox, if I then press Command-N (File> New Message) or hit the "New Message" button on the toolbar, my email will be sent from that Gmail account. That's kinda confusing and weird, I think.
Now of course, you can toggle the "From" box on your outgoing message to send from a different account every time if you want to…
…but I prefer to change the default. I don't know about you, but I rarely pay attention to what I've selected before I start a new message! Plus, if I had to change that "From" box every single time I sent an email, you'd probably hear about me on the news.
Anyway, here's how you do it. Choose Mail> Preferences, and then click on the "Composing" tab. About halfway down the window, you'll see Send new messages from.
Toggle that to whichever account you want to use most of the time, and you're done!
You can choose which one to send from by default on your iOS devices, as well. That preference is within Settings> Mail, Contacts, Calendars, and it's labeled "Default Account."
As it notes in the screenshot above, changing this will affect any emails you send from outside of iOS Mail, too. So if you share a link with someone from the Safari app, it'll use whichever account you've picked here. Just like on your Mac, you can then click on the "From" field on your iOS device to select a different account to send from if you need to.
That is one blurry image, man. I think I need about twelve fewer email accounts.