1. Are you sincere or sincere (all letters)

Best wishes. I wish you happiness forever (personal letter)

3. Deep feeling, your deep feeling, love and loveliness (to close friends and family)

4. Yours is faithful and yours is sincere (official letter)

In a formal letter, write to a group. You can use: Trully yours, faithfully yours.

English letters with extended information are one of the most commonly used applied styles. For an ordinary private letter, it usually consists of five parts: header, date, title, content (text, conclusion) and signature.

Sometimes you can see words like Re: or Subject: (cause) between the title and the text. Generally in the middle of the stationery, it can also be aligned with the "address". You should also add a horizontal line at the bottom to attract the reader's attention and make it easier for the recipient to understand the main content of the letter before reading it. Reasons are generally used in official letters and can also be omitted.

References:

Baidu encyclopedia-English letters