Subscribed Message text garbled

I'm considering to send our digest mail in Japanese with your Subscribed message by Mail(SBM).
But all Subscribed Digest Mail are garbled... And No image in it.

I made&wrote some language file "subscribe-by-email-ja.po" and ''".
And I rewrote some message in Japanese at WP admin's SBM setting page,
and "preview"is fine!!

And I checked myTheme/subscribe-by-email/header.php.
But I could not discover the cause.