When answering a letter, you should always be polite. The following sentences may be useful for you to expresss thanks to those who wrote to you.

Thank you very much (very, very much) (ever so much) (most sincerely) (indeed) (from the bottom of my heart).
很(非常)(非常非常)(最真誠地)(確實)(衷心)感謝您。

Many thanks for your kind and warm letter.
感謝您友好而熱情的來信。

Thanks a million (ever so much).
萬分(非常)感謝。

Please accept ( I wish to express )my sincere (grateful) (profound) appreciation for ...
請接受(致以)真摯的(衷心的)(深切的)感謝……

I sincerely (deeply) (warmly) appreciate ...
我真摯的(深深的)(熱情的)感謝……

I am very sincerely (most) (truly) grateful to you for ...
為了……，我非常真摯的(深深的)(真誠的)感謝您。

There is nothing more important (satisfying) (gratifying) to me than to receive one of your letters.
再也沒有比收到您的來信更使我覺得重要(快慰)(感激)了。

Your letters are so much fun (comfort) (entertainment) (company).
您的來信充滿了樂趣(給了很大安慰)(帶來了歡樂)(使我不感寂寞)。

Your most courteous (considerate)(delightful)letter...
您那彬彬有禮(體貼入微)(令人欣慰)的來信……

I can not tell you how much your letter delighted(relieved)(amused)(enchanted)me.
我無法告訴您，您的來信使我多么高興(寬慰)(覺得有趣)(陶醉)。