Genitive: The letter's content
Письма контент
1) It’s correct but sounds unusual for everyday speech. Usually "object" is first, and "owner" is second: so it should be "Контент письма".
2) The "контент" is loan word and it’s used in technical text. Use the word "содержание" instead "контент" in everyday speech.
Should be: Содержание письма.


Dative: Give the letter to me
Дай письму меня
Should be: Дай письмо мне.


Instrumental: I replied by letter
Я отвечал через письмом.
1) "Через" means "through", not "by".
2) Replied (past simple), so you should use perfective aspect.
Should be: Я ответил письмом
or... Я ответил посредством (при помощи) письма... However another case is used in this constructions