I'm not sure I've heard of this method before. I can't tell if you're directly signing on with some special email address as an ID, or an email is received by certain websites and logs you in that way (or authorizes you to log in for a short period of time). Could you provide a link to a description of the steps involved?
Regarding security levels, a chain is only as strong as its weakest link. Any unencrypted email could be intercepted and read, and then spoofed. Email addresses are out in the open (not encrypted). Without understanding more about how this login system is supposed to work, I can't say anything more than that.