When sending emails to a group, some ISPs require that their customer be listed in the "To" line, not the Copy-To line or the Blind Copy-To line.
If they don't see their customer listed in the To line, they can reject the email.

Likewise, some ISPs will limit the number of Copy-To's to 25 while other ISPs will limit the number of Copy-To's to 50.  There is no industry standard.
If you're sending out to more than 25 people at a time, some ISPs will detect that and reject your email.

The solutions:

 - Limit your email to 10 or 15 recipients at a time.

 - If you get a bounce-back from one or two recipients, try sending those out again individually.  See if that makes a difference.

 - Use a 1USA Mailing List instead.  If you frequently send out to a Group, 1USA Subscribers may get a free GroupList.
The 1USA GroupList server sends out individual emails.  Your recipients will not know who else is on the list.  You can manage your list via email !!
Contact the 1USA Webmasters to setup a list for you.