This is a bug report.
After updating to 10.12.4 from 10.12.3,
character corruption of some of the received mails
occurred in Mail.app (ver. 10.3 (3273)).
The mails with character corruption were surely
readable before the update.
IMAC (RETINA 5K, 27-INCH, LATE 2015), macOS Sierra (10.12.4), null
Are these emails in Japanese?
Are these old emails, or do you have the problem with new emails that you receive right now?
I've just reported it as a bug report through the link.
I hope it is solved soon.
Report:
I upgraded to macOS Sierra 10.12.5, but the problem still remains...
Nobody will ever be fixing anything in Sierra. Are you running that? Or High Sierra?
This issue is in both Sierra and High Sierra.
Hope to be fixed in High Sierra or later version.
Please post the details of your problem with a screen shot in the high sierra forum so it can be dealt with for an os version which is still fixable.
character corruption in Mail in macOS 10.12.4
