This is for Windows users only.
The only way Windows users could successfully export multiple photos directly to an email attachment from Lightroom was using a (free) third-party app called MAPI Mailer created by Steve Sutherland. Well, I just learned (thanks to Lightroom-News.com) that Steve has upgraded MAPI to a full fledged Export Plugin, which removes the previous limitation on how many photos could be attached at a time.
Steve has even created a set of instructions on how to upgrade your old MAPI Mailer export presets (if you have them) to work with the new plugin. Thanks Steve!