The only problem with e-mails is that the data on them can become corrupted and there's always the chance that you may be accused of falsifying the information. I'm not saying e-mails can't be used to prove something is legally yours, but it might not be the easiest option.
What I did with my ms before I started querying was I sent a copy of it to myself by registered post, then I left it in my desk, unopened. In Ireland, registered post is recorded and the delivery archived by the government, with the envelope sealed by the post office with a label that lists a unique serial number.
If there's ever a case where you need to prove you wrote the work, you take the envelope to a solicitor and have it opened in their presence, and they can verify to a court that the work was yours first. I don't know if you have something similar in the US, but here it's about the easiest way to protect your intellectual property.
With regard to beta readers or crit partners, though, you should only ever send your work to people you really do trust.