To search exclusively for mails between two mailboxes this is likely best achieved via keyword expression:
the syntax would be along the lines of
Email > Keyword Expression > matches > to:("Mailbox1" OR "Mailbox2") AND
Email > Keyword Expression > matches > from:("Mailbox1" OR "Mailbox2")
Where Mailbox1 and Mailbox2 are the email addresses for the mailboxes in question.
This will find mails between the two mailboxes using the From/To/CC fields, but not BCC.
It will also show messages where either Mailbox1 or Mailbox2 are includes in a distribution list in those fields due to group expansion.Go to the full post