What is the internal mail?
In very large companies, organizations, hospitals etc., even if a mailman delivered the sacks of mail to the company himself, he is not going to go from desk to desk, personally handing letters to individuals. Instead, such companies etc have a mailroom, where all mail addressed to the company is sorted into the various departments. This mailroom attendant then delivers the mail to each of these departments; and then a secretary probably goes through the mail delivered to her department, and places it into each person's pigeon-hole.
This whole process, from company mail room to appearing in one's pigeon-hole, is called the 'internal mail'.
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