I have been using Windows  7 and Windows Live Mail 2012 for many years.  Outgoing settings have been as follows: 

Outgoing mail: smtp-mail.outlook.com

Outgoing SMTP server: 587

Authentication required: This server requires a secure connection 

However, with NO changes to the above, since a few days ago emails attachments, e.g. pdf files,  have been failing with following error message:

An unknown error has occurred.

Subject 'Letter.pdf [01/12]'

Server Error: 554

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

Server: 'smtp-mail.outlook.com'

Windows Live Mail Error ID: 0x800CCC6F

Protocol: SMTP

Port: 587

Secure(SSL): Yes

The Message Delivery Failure message:

Delivery to the following recipients failed permanently:

   * xxxxxxx@hotmail.com

Reason: This is the mail system at host know-smtprelay-4-imp.
 
I am sorry to have to inform you that your message, "Letter.pdf [01/13]", could not be delivered to xxxxxxx@hotmail.com.
 
 The remote server returned the below error when attempting delivery:
 
554:554 5.6.1 Messages of type message/partial are not supported [BY2NAM03FT046.eop-NAM03.prod.protection.outlook.com]

I have tried turning off, "This server requires a secure connection", but that did not solve the problem.

Solution suggestions will be mightily appreciated, thanks.