Cannot send a group email with attachments

[math whiz]

I cannot send a group email with attachments, but I could less than 8 hours
ago. This is the error message I received:

The message could not be sent. The authentication setting might not be
correct for your outgoing e-mail [SMTP] server. For help solving this
problem, go to Help, search for "Troubleshoot Windows Mail", and read the
"I'm having problems sending e-mail" section. If you need help determining
the proper server settings, please contact your e-mail service provider.

The rejected e-mail address was (someone's email address) Subject
'Registration for 2009 - 2010', Account: 'pop-server.mi.rr.com', Server:
'smtp-server.mi.rr.com', Protocol: SMTP, Server Response: '550 Invalid
recipient: (someone's email address)', Port: 25, Secure(SSL): No, Server
Error: 550, Error Number: 0x800CCC79

 

[Gary VanderMolen]

Do you get this error with every recipient? Since you didn't include
the rejected email address, I can't check it for validity.