This one has come up for me a couple times in various places. The most recent was when I clicked on the "update your contact info" link in an existing reservation:

E-mail address invalid
The e-mail address entered is invalid. This user ID (before the "@" symbol) must be at least two characters.

My email is "[email protected]". I assure you email addresses can be just a single character. You should validate email addresses based on the published RFCs, not what the developer thinks is reasonable.