Email misconversion of 8bit encoding after version154

So long as I investigated, the problem is, After version154, received emails in Japanese encoded by 8bit causes misconversion (mojibake). Emails encoded by base64 is not misconverted. Only 8bit encoded shows the problem. Version153.5 is no problem. Version154 and newer shows the problem. Here are OK/NG by sender/receiver combination. Sender version154 8bit - receiver version154 - NG Sender version154 base64 - receiver version154 - OK Sender version154 8bit - receiver version153 - OK Sender version154 base64 - receiver version153 - OK Sender version153 8bit - receiver version154 - OK Sender version153 base64 - receiver version154 - OK Emails encoded by base64 is OK, but some senders can encode by only 8bit. I cannot find how they can use base64. It would be helpful to know to use base64 encoding in version154 (or later). However, the best way is that version154 (or later) can read 8bit encoded email in Japanese. I would like to thank anyone who help the problem.