大衛的部落格

把IdSMTP傳送郵件的內容改用HTML格式發現原本的HTMP標籤會截斷或是斷行，經過測試如果999各字元長度都不給予CRLF就會產生這樣的問題。收到的郵件格式會異常，查了一下發現RFC 5322 Internet Message Format就有寫明限制了: 

2.1.1.  Line Length Limits

   There are two limits that this specification places on the number of
   characters in a line.  Each line of characters MUST be no more than
   998 characters, and SHOULD be no more than 78 characters, excluding
   the CRLF.

   The 998 character limit is due to limitations in many implementations
   that send, receive, or store IMF messages which simply cannot handle
   more than 998 characters on a line.  Receiving implementations would
   do well to handle an arbitrarily large number of characters in a line
   for robustness sake.  However, there are so many implementations that
   (in compliance with the transport requirements of [RFC5321]) do not
   accept messages containing more than 1000 characters including the CR
   and LF per line, it is important for implementations not to create
   such messages.

   The more conservative 78 character recommendation is to accommodate
   the many implementations of user interfaces that display these
   messages which may truncate, or disastrously wrap, the display of
   more than 78 characters per line, in spite of the fact that such
   implementations are non-conformant to the intent of this
   specification (and that of [RFC5321] if they actually cause
   information to be lost).  Again, even though this limitation is put
   on messages, it is incumbent upon implementations that display
   messages to handle an arbitrarily large number of characters in a
   line (certainly at least up to the 998 character limit) for the sake
   of robustness.

文件建議每行不應該超過78各字，比較強的用詞是每行一定不可以超過998各字(中文可能是算2各字元，這個我沒有測試)，平時還是要去研究一下各項協定的限制，不然盲測有點太花時間。所以保險作法就是每行78字元個就加上CRLF。