Question: 

I would like to create a report that tracks the number of e-mails sent from one set of local mailbox users to another set of local mailbox users - or from one mailbox user to another one.

For example:
Group-1 = user-A, user-B, user-C
Group-2 = user-X, user-Y, user-Z

The report would show something similar to the following:
User-A received 10 messages from mailboxes in Group 2;
User-B received 12 messages from mailboxes in Group 2;
User-C received 17 messages from mailboxes in Group 2.

Answer: 

You can use the Find Specific Messages report template, that you will find on the Templates > Mailbox Traffic menu, and apply the following settings:

• Period tab: choose a period for your report.
• Selection tab: Recipient type: Mailbox; Recipients by: Display name (or Server, Organizational Unit, Department...). This is where your will select  Group-1.
• Correspondent tab: Recipient type: Mailbox; Mailboxes by: Display name (or Server, Organizational Unit, Department...). This is where your will select  Group-2.
• Group tab: No grouping.
• Filter tab: not used.
• Options tab: Sort by, Message count, Sent, Descending; Assign sent on behalf messages, To the delegate's mailbox.
• Content tab: Message traffic summary by recipient; Include detail of correspondents.
• Presentation tab: Your own choice.
• Output tab: Your own choice (Screen, Printer, Disk file, Email recipient, Sharepoint, Database table).