Sendmail 8.7 has a new annoying "feature": if you don't protect yourself, it may start converting your nice 8-bit mails to QP or BASE64. This can happen if you reuse old configuration files from 8.6.
There are probably many different ways to correct this behaviour. The one I used was to insert the mailer-flag "8" on all mailers. See below for an example of how it looks at the sendmail.cf level:
Mrelay, P=[IPC], F=mDFMuXa8, S=11/31, R=61, E=\r\n,
L=2040, A=IPC $h
The configuration files for sendmail 8.7 defines an smtp8 mailer, which you can use instead of smtp, that has this flag set.