When you tap send, you'll see a screen that tells you how big the current message is and asks you if you want to scale your images down to reduce the size of the message. Just tap the desired option (Small, Medium, Large, Actual Size, or Cancel).

One of the most welcome improvements to Mail is that you can now configure a different signature for each of your email accounts. Go to Settings -> Mail, Contacts, Calendars -> Signatures, and then choose Per Account. This enables a separate signature field for each email account you've set up on your device. Type or paste your preferred signatures, and Mail will automatically append, to each outgoing email message, the appropriate signature for the sending account.

You no longer force Mail to check for new messages by tapping a clearly visible refresh button at the bottom of the screen. Instead, Apple has in iOS 6 adopted the trendy "pull to refresh" approach first found in the original Tweetie Twitter client for iOS. This means that when you want to force an email check, you must first navigate to a mailbox view or the Mailboxes screen--you can't force a refresh while viewing a message, as you could in iOS 5. Then you swipe the screen downward as if you were trying to access the Search field while in a mailbox. Once you see a little refresh button (the one with the circular arrow) at the top of the screen, keep swiping until that button stretches down and then "snaps back" to a progress indicator.