It means that the server does not recognize it as a valid email address.
You
are dealing with the literal-mindedness of a computer.

This begs the question: why? I send to this address all the time. This
has happened with other addresses. They work, then they create this error
message, and then at a later time they work again.

Could be that the recipient's mailbox is full, among other
things.

The RECIPIENT's e-mail box being full would not prevent me from sending an
e-mail. A full mailbox at the recipient's end just results in my later
getting back a message saying that my e-mail could not be delivered. That's
not what happens here. Here, I cannot even Send. Thus if I am sending an
e-mail with 10 names in the cc line, the error with that one name prevents
the e-mail from being sent to any of those names. It remains in my Outbox.

AT&T Worldnet is not as generous as at&t Yahoo! HSI when it comes to
mailbox
size. 25 MBytes, as compared with 2 GBytes. It doesn't take a lot of email
with large attachments to fill a 25MB mailbox; and AT&T Worldnet will stop
accepting email when a mailbox is full.

No, there's vast amounts of space at my mailbox, that can't be the reason.
How could a single, small e-mail represent some kind of overloading in any
case?