改Grammer (不用翻譯網站)

1)的句式邏輯和用詞的前後都是錯的。
2)前部份ok(其他人回答都當作是郵寄，但你指明是你把合約投入對方的信箱而不寄給對方，你是該大廈的管理公司吧，所以你寫的是對的。
後部份亦差不多了，只把句子改為：Please return after signing.
你會見過很多人會寫Please mail it back after signing.文法上沒錯，但mail back是中文「寄回來給我」，英文人甚少會這樣寫的，因為return便是最簡潔的寫/講法。

Please sign and return it to me.

I have sent you the renewed contract by mail, please return it to me after signed.

我已將renewal合約放入你信箱，簽署後，請寄回給我。

I have put a copy of the renewal agreement into your maibox. Please send it back to me after signing it.

I have sent you the renewed contract by post, please kindly sent back after signing it.

I have sent you the renewed contract by post, please return it to me when you have sign it.

2011-09-06 17:02:24 補充：
When you have "sign" it

Should be "signed"

I have sent to your mailbox for the renewal. Please mail us after sign.