【IT Notes】Postfix的郵件轉寄限制

 最近常遇到一個郵件轉寄的狀況，公司mail server裡面有幾個群組信箱，用的是新增forward的方式將外部寄進來的信轉寄到其他用戶，這是很方便的設計，但是最近當轉寄人員太多的時候，就發生名單中某些用戶沒有收到，而且斷在一些很奇怪的地方，一開始以為是收件者帳號有問題，但一旦更換名單的順序，就會變成其他人收不到信，明顯已經看出來每次都是斷在同一個地方，而且斷字後面的其他收件人依然有收到信，就只有被斷字的帳號被拆成兩次寄送，最後被系統打回來說找不這兩個收件帳號。

後來我就將轉寄清單複製到word上面，然後找到斷字的位置，用word的計算字數功能，最後查出每次斷字的字元數都在2048，可以得知forward轉寄的字元數被限制在2048的字元，一但超過它會自動拆開，並當成另一個寄件者再寄。看到2048這個這麼剛好得數字就可以猜到一定是某設定值要修改，於是就google一下postfix上有關2048的設定有哪些。

錯誤訊息如下：

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
                   The mail system
<i@piapp.com.tw> (expanded from <pi@piapp.com.tw>): unknown user: "i"
<rene.kuo@piapp.com.tw> (expanded from <pi@piapp.com.tw>): unknown user:
    "rene.kuo"

第二次也收到一樣訊息

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
                   The mail system

<a@piapp.com.tw> (expanded from <pi@piapp.com.tw>): unknown user: "a"<nna.shen@piapp.com.tw> (expanded from <pi@piapp.com.tw>): unknown user:
    "nna.shen"

解決方式：

google後找到此一設定值

line_length_limit (default: 2048)

Upon input, long lines are chopped up into pieces of at most this length; upon delivery, long lines are reconstructed.

接下來到/etc/postfix/main.cf上面多加一行，line_length_limit＝4096，把字元限制長度多增加一倍，這樣問題就解決了。