An unknown error has occurred.

Subject 'Kinsmen Fanshawe Sugar Bush 2017 Season is about to begin!'
Server Error: 550
Server Response: 550 5.3.4 552 5.3.4 Header size exceeds fixed maximum size
Server: 'smtp.live.com'
Windows Live Mail Error ID: 0x800CCC69
Protocol: SMTP
Port: 587
Secure(SSL): Yes

works for basic mailings but this I had to email to about 30 volunteers.

any ideas?