请原谅我的回信延迟……，等。

1. Please excuse my late reply to your very friendly letter of March 1.

2. I hope you will forgive me for not having written you for so long.

3. I hope you will excuse me for not having replied to you until today.

4. I humbly apologize you for my delay in answering to your kind letter of May 5.

5. I have to (must) apologize you for not answering your letter in time.

6. I must ask you to kindly accept our excuses, late as they are.