Formail will detect if the Message-ID of the current message has
already been seen using an idcache file of approximately maxlen
size. If not splitting, it will return success if a duplicate has
been found. If splitting, it will not output duplicate messages.
If used in conjunction with -r, formail will look at the mail
address of the envelope sender instead at the Message-ID.
if the Message-ID of the current message has already been seen using an idcache file of approximately maxlen size
如果當前郵件的message-ID看起來和idcache文件當中的差不多。
If not splitting, it will return success if a duplicate has been found.
如果不是分割的，在發現重複郵件的時候它將返回一個成功
If splitting, it will not output duplicate messages.
如果是分割的，它將不輸出重複郵件。