Passing along multiple emails at once is pretty simple (and much faster than forwarding tons of stuff one message at a time, let me tell you). So if you need to send someone the entire history of correspondence on a project, for example, here’s how you’ll do that. First, open Mail. Then either do a search for the messages you need or open the folder they’re in. You’ll have to select them all, so there are a couple of keys that’ll be your friends—Shift and Command. If you click on one email and then hold down Shift and click on another, you’ll select everything in between them. Alternatively, you can hold down Command and click on messages, and your Mac will select everything you click on (even if the emails aren’t sitting next to one another).
Once you’ve got stuff highlighted, you’ll have a couple of choices. First, you could just click the “Forward” button in your toolbar (or press Shift-Command-F), and that’ll start a new email with all of the items you selected in the body of the message. Secondly, you could instead forward the selected emails as attachments, which’ll make them come out as…well, as attachments on the other end.
You’ll do that by selecting the emails and then picking Message > Forward as Attachment from the menus at the top…
…or by right-clicking on one of the emails and choosing that same option from the contextual menu.
Whichever method you choose, though, your recipient should be able to look through everything you’ve sent him or her, and you’ll be spared the tedium of forwarding the messages individually! Hooray for that.