Emails are sent to an email address. Each email address is then connected to an email account, also called an email user. 
Each email account can have any number of email addresses attached to it. For example my email account myaccount01 has
the email addresses paddy.murphy@mydomain.com and paddy@mydomain.com connected to it. So, when I download emails from my 
account, I am downloading emails sent to either of these two addresses.
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So, how many email accounts do you need? The best way to think of it is to think about how many different PCs will be downloading emails.
Since each PC should have its own email account, the number of PCs you will be using will correspond to the number of email accounts 
you'll need.