It always bugged me that whenever I’m deleting an e-mail in Apple’s mail client Mail.app another mail is selected and subsequently marked as read.
Cmd + Shift + U became a good friend of mine. (It marks a mail as unread.)
TurePreview is a lightweight Mail.app plugin which gives you all the time you need to change the selected mail and thus prevent a mail from being marked as read. This is very handy if you keep unread mails as a have-to-reply reminder in your inbox. (This is the opposite of GTD—I know and I do it just for mails I have to reply to.)
TurePreview can set a general delay or delays on a per account basis. Whatever floats your boat.