[giantrobot]

In practical terms e-mail addresses are case insensitive. So if on account creation your normalize the address (lower case, trim white space) and send a verification e-mail and they successfully verify you can safely derive an ID from that normalized address. It won't matter later if autocorrect tries a mixed case address since you normalize and compare it on the back end.

If you run into a case where their e-mail server enforces case sensitivity they have bigger problems to deal with. E-mail has long been a system that requires loose adherence to the specs.