E-mail client: Windows Live Mail

OS: Windows 7 64 bit SP 1

Until recently, my e-mail worked great. then all of a sudden I get the following error message only when I try to send out e-mail:

An unknown error has occurred.

Subject 'Emailing: FedwarBridgehead90.txt'
Server Error: 550
Server Response: 550 5.2.0 wWBx1m00E5VZmhB05WBxgX Message identified as SPAM - Please visit http://www.charter.com/postmaster E5110
Server: 'smtp.charter.net'
Windows Live Mail Error ID: 0x800CCC69
Protocol: SMTP
Port: 25
Secure(SSL): No

I can receive e-mail OK, I just cant send out e-mail. It occurs with e-mail to different addresses, and with and without attachments. I have not downloaded any updates recently. Any help would be appreciated.