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what do you include in the address's in a formal.. eg, postcode, date?

[Anglika]

Dop you mean the address on the envelope, of the first page of the letter?

The address >> Recipient's name, first line of address, second line of address, name of city, post code. one lone of space, then date.

The first page of a letter: If not on company letterhead, then your address in the top right corner.

The recipient and his/her address two lines down and on the left. First line of letter ( greeting) one or two lines below address, and on left.

First paragraph of letter: one line below greeting, and on left.

See here: Formal Letter Writing - Articles - UsingEnglish.com