When trying to send photos from photo gallery I receive this message:

The message could not be sent because it could not be split into smaller parts.

Subject 'Your garden... {C8440536-CBE3-4D74-BC28-A6CF905F0784}.wpostx [01/38] [2/2]'
Server: 'relay.plus.net'
Windows Live Mail Error ID: 0x800420CA
Protocol: SMTP
Port: 25
Secure(SSL): No

What does this mean, please, and how do I resolve the problem?