How many emails received externally?

Hello,

We have 5 mailboxes that are used for invoicing. every email received externally is an invoice.

My manager wants to know how many emails received in the first 3 months of this year per mailbox.

Is there a software that can calculate the number of emails received externally within a certain period?

How about PowerShell? is there a command that can do this?

Thanks in advance.