This just started yesterday.  Though I continue to receive lots of email messages, I am now unable to send them.  When I try to send an email, I get the following error message:

An unknown error has occurred.

Server Error: 554

Server Response: 554 5.6.1 Messages of type message/partial are not supported

Server: 'smtp.live.com'

Windows Live Mail Error ID: 0x800CCC6F

Protocol: SMTP

Port: 587

Secure(SSL): Yes

I omitted the subject line from this error message, because the message is of a personal nature.

My computer still runs on Windows 7, and I am using Windows Live Mail 2012.  Also, I have checked, and the Mail Shield in my Avast antivirus protection program is not only turned off, but uninstalled.  So that shouldn’t be a problem.

When I made my first attempt at sending a message, I had included a few attachments.  Thinking that perhaps I might be trying to send too many attachments with too much data, I decided to try sending four emails, with just one attachment included with each.  But this didn’t help.

Is there anything else that I should be checking, or anything else that I need to change or do differently to be able to send emails again?

Thanks in advance for your help.

Edit:  I just tried to send an email with no attachments, and I got the exact same error messages.   So apparently the attachments weren't the cause of the problem.