Yes, there can be limits setup in the server but there are so many places
where messages could be limited that it would be too difficult to tell you
exactly where things got cut off.
Also, you cannot be 100% certain that the messages did not send since
some users may have the messages filtered as spam or otherwise trapped
and not delivered for reasons other than what you thought.
There is also the possibility for large mail sends that the messages are
actually still being sent. It can take a good deal of time to flush a
mailing list run from the mail queues for a large bulk mail send.
I would look at your mail logs (stored in /var/log) and trace how the
messages are sent and if you received any errors or failures in sending
which could explain why some didn't get the message yet.
I would also get
Chirpy's mail queue manager and
mail manager to give you
a better view of what is going on with the mail being sent from your server.
You can also check your "Tweak Settings" and "Exim Configuration" in WHM
to see if you have any sending limits imposed and increase those numbers.