Sending though cox.net

I have a warning message on the toolbar,

[SMTP]Cannot send message (‘From’ field address ‘k***72@cox.net’ not accepted due to the following reason:
"k***72@cox.net sender rejected
")

The thing is, I have no email in the Outbox, so this error shouldn’t be there.

I redirected the From field from the warning dialog and already sent the message.

Outgoing messages are stored in the Local Folders Outbox. If Local Folders are not visible, you can enable them in Menu > Settings > General > Show Local Folders.

Yes, I checked that.