FormalGreeting:
Dear Ms. White, Dear Sir/Madam,
Body:
We are writing to inform you that ...
With regards to your letter from [date] ...
Closing:
If you have any questions, don't hesitate to contact us.
Yours sincerely, / Yours faithfully,
InformalGreeting:
Hi Ben, Dear Ben,
Body:
How are you?
Many thanks for your (recent/last) letter
Would you like to go to the cinema with me.
Closing:
Anyway, I must go and get on with my work!
Look forward to seeing you again.
Lots of love,