Suddenly can not send email from windows mail

[marsky]

An unknown error has occurred. Subject 'Cute pictures', Account:
'pop.mindspring.com', Server: 'smtp.earthlink.net', Protocol: SMTP, Server
Response: '550 Syntax error in 'To' header: "@" or "." expected after
"Undisclosed-Recipient": failing address is: <Undisclosed-Recipient:;>',
Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC69

 

[Gary VanderMolen]

The EarthLink server is very fussy in that it doesn't allow a blank To line.
When you put all the recipients on the BCC line, put your own email address
on the To line.