Here is a test case:

Attached is file inbox-mixed-charsets which contains 2 messages, one
using utf-8, one with iso-latin-1 charsets.  Follow the steps:

C-u rmail inbox-mixed-charsets

You look now at the second message.  Save rmail file, kill buffer and
visit again:

s
C-x k
C-u rmail inbox-mixed-charsets

First message looks like this:

... ae=\344 oe=\366 ...