[ballenf]

You could let people pass the username/password in as part of the email tag, like:

[e-mail]

As others have clarified, only for complete throwaway accounts obviously. I tested '-' and '!' and those characters appear acceptable as the delimiter. ':' gives a very strange error on send when using gmail.com. Also imagine you'd lose some letter casing along the way.

Really just a fun and terrible idea!