All the information required for the person you're writing to to be able to reply, so a full postal address. If you're writing to someone in your country, there's not need to include that, but if you're writing to someone abroad, then include the country.
With letters, I think it is good to be clear and brief, so I don't see a need for introductions. I do the following:
1 the purpose of the letter (complaint, application, request for information, etc)
2 details and information needed
3 action I want the other person to take
4 thanking them and ending the letter
Other people may do things differently.
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