The following non-delivery report:

This message wasn't delivered to anyone because there are too many recipients
Too many recipients

means that the number of recipients exceeds the limit set on the sender's side.

In the Diagnostic information for administrators section recipients are listed with the following error message:

#550 5.5.3 RESOLVER.ADR.RecipLimit; too many recipients ##
5.5.3 RESOLVER.ADR.RecipLimit

By default, the number of recipients limit is set to the maximum for each user on the account. To change this setting, navigate to Services > Mailboxes > Exchange Mailboxes and click on the user in question. Then go to Advanced Settings > Limits:

Note: as a workaround, you can create a Distribution List and add all (or some) recipients as members. On Exchange, the distribution list is viewed as one (1) recipient regardless of the number of members on the list.

Important: if the number of recipients exceeds 200, this may cause delivery delays. Also, such messages can be considered spam and rejected by 3rd party mail systems. Intermedia does not recommend to send to more than 200 recipients at a time.