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 ''subscribe-by-email-ja.mo".
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.