The 'correspondents' are those who sent messages to, or received messages from your selection of recipients.
The vast majority of traffic report templates include a Correspondents tab. This feature lets you filter the correspondents of the recipients previously selected in the Selection tab.
The most usually available option allow you to select correspondents by address type.
example
You may want to select correspondents from specific SMTP domains, for example the gmail.com, yahoo and hotmail correspondents of the selection of mailboxes you have just checked in the Selection tab.