Default connection limiting?

[MRob]

I have group of user behind single WAN using mine Postfix submission 
service. Sometimes they cann't connect but I dont know why. I thought 
its cause that Postfix has default connection maximum from single IP 
source, is this true?

* What is error/fail message in logs which I could find to verify I have 
that problem?

* What is relevant postconf setting I will have read more about?

Thank you!