When you are mailing a busy person -- or anyone who values their time and doesn't like long emails -- compose your draft and check it on your smartphone first. That's an easy way to tell if it's too long or not.
Photo by bradleypjohnson
That suggestion comes courtesy of 99U. What appears digestible" on a desktop computer looks like an epic poem on a mobile phone. Is it still concise on an iPhone? Once you check, you will probably realise there are a few more things you can remove. Edit your email again, and then send.
For the best effect, get to the point of why you are contacting that person in the first couple of sentences. We have plenty of other tips on how to be concise in emails.
The rest of the 99U post is worth checking out for some pretty solid advice on requesting favours through emails.