MULITIPLE PCS WITH POP3 EMAIL ARE MANY DOORS FOR A VIRUS TO ENTER YOUR BUSINESS. A SERVER BASED EMAIL SOLUTION IS ONLY ONE DOOR....
Basically there are two email systems, Server based and POP3 / IMAP.
POP3 / IMAP is the norm for home users or businesses that do not have a server and collect their emails direct from their ISP / Domain provider.
One system for a Server based solution is Microsoft Exchange Server, alternatively it maybe via a ' CLOUD' solution.