can't sent email, 64 bit error 552 5.2.3 Port 587 have comcast.net

I'm not able to send emails. Error message reads: "Account: mail.comcast.net, server: smtp.comcast.net, Protocol: SMTP, Server Response: 552 5.2.3, DATA size exceeds maximum permitted, PORT 587, Secure (SSL): No Server Error: 552, Error Number: 0x800CCC6D

I just set-up the email account on this computer (Vista 64 bit). I am able to receive OK. HELP!