one contact (email address) can only be linked to one account

one contact (email address) can only be linked to one account

Hi have an issue with email.       My delemma is that I have several (80+) accounts all within the same company, we will call this company xyz.    Comany xyz has MANY satellite offices around the country and its own internal help desk that all calls must flow through.   
XYZ's help deks calls my help desk for support and my help desk must email back xyz's helpdesk with status etc.
 
xys'z  email address is xyz@xyz.com       
 
My problem is this.   since XYZ has so many sub offices and I need to report on calls taken for each sub office,  I need the ability to use the email address xyz@xyz.com  for each of these accounts as the contact.  
 
How can I use the same email - contact for XYZ's Many sub accounts???? 
 
 
Doug

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