If you have several client installations on your network, all using the same data, complications may arise if more than one collects mail via POP3. Multiple simultaneous connections to one POP3 mailbox can cause lockouts, or duplicate messages being downloaded.

It is best to configure just one machine to do the mail collections, and disable the others. 

However, be cautious:  ensure that the MailCOPA User doing the collections will collect mail for all of the users who need it collecting

For example, if user John does the collections, but either:

Paul does not allow others to collect his mail

John is not configured to collect mail for Paul, even though Paul has allowed his mail to be collected by others

In either of the above scenarios, Paul would have to collect his own mail