Cannot send e-mail

It took quite a while to downlowd my e-mails.
When I attempt to send an e-mail, I get the error "Sending of the message failed.
The message could not be sent because the connection to Outgoing server (SMTP) smtp.comcast.net was lost in the middle of the transaction. Try again."

I get this error no matter how many times I try to send, or who the recipient it