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Letter writing is an essential part of business communication. A letter is a written message from one party to another.
So, the letter delivering process:
Sender writes letter and places it in an envelope on which the recipient's address is written.
Sender buys a postage stamp and attaches it to the front of the envelope on the top right corner on the front of the envelope.
Sender puts their letter in a postbox and does nothing more/
The national postal service of the sender's country empties the postbox and takes all the contents to the regional sorting office.
The sorting office then sorts each letter by address and postcode and delivers the letters destined for a particular area to that area's post office.
The local post office dispatches the letters to their delivery personnel who deliver them to the proper addresses.

This whole process, depending on how far the sender is from the recipient, can take anywhere from a day to 3–4 weeks. International mail is sent via trains and airplanes to other countries.