It's many years since I administered email, so I could well be mistaken, 
but I think this is a result of spam-filtering products being a little 
over-aggressive. Any message with numerous recipients could conceivably 
be taken by software to be spam, There are likely more than a few 
software products in use which produce somewhat different results, 
resulting in a lack of consistency. You can always talk to your your 
email provider, but in most cases, tier one support will have no idea 
how to  resolve your issue, and the email specialists won't even want to 
talk to you about the products or methods they use to filter email.

FWIW
Don