I like the fact that united.com aka .bomb accepts '+' in the email address.
It would be very useful in a case like you described.
Let's say you have an email account calld [email protected], if you email [email protected] the email should still end up in the [email protected] inbox (some email provider may not support this but I know that gmail support this type of address).
So you can enter

[email protected]
[email protected]
[email protected]
[email protected]

on united.com, you can access all emails in one mailbox, [email protected]
You can also use this type of email address for login or password retrieval on united.com ^