I have free email service account (do not show here which one to avoid advertising) with the both methods of access : POP3 and via Web

I have setup POP3 access in my Windows Live Mail (WLM) and it works fine, but when I send a message to one of the particular domain email accounts via WLM/POP3 it is bounced with the following error message:

Remote host said:
552 5.2.0 6vqn1p02j1CGm2A09vqpgd automated process detected unsolicited content

[BODY]

But, if I send the same message with the same, body, subject etc, from the same PC via Web it is delivered well.

Question is why some email servers don't like messages send via WLM/POP3?

It is obvious the problem is not with my PC, ISP/IP, antivirus, message body or subject etc.