[Edited: the below no longer works. I do not know of a sure way to find a Twitter account by email. However, I do know how to find an account on LinkedIn – for long lists of emails at a time – check it out!]
I will continue “the series” by explaining how to find the Twitter ID from an email addresses. In fact, though the method requires several steps to follow, the process will discover the Twitter IDs for ~500 email addresses at a time.
Step 1. Create a Gmail account and upload the 500 email addresses in question. To clarify, this account should have exactly these 500 contacts (or fewer).
Step 2. Go to https://twitter.com/who_to_follow/import and point to that Gmail account.
Voila! Those IDs registered with the imported contacts will be identified. The display will look like this:
By the way, this is a way to mass-follow people, too (!)