The input file is valid UTF-8, but this is what your mail client reads:
...
Subject: [...] acceptÃ=?UTF-8?Q?=A9?=e.
...
Let's try to interpret your resource with different encodings:
Votre/vos manifestations(s) ont été acceptée(s). // CP-1252
Votre/vos manifestations(s) ont été acceptée(s). // UTF-8
The Ã
is written verbatim in the field body, while U+00A9
is the code point for the copyright symbol. We can't tell for sure, but either you do something wrong with the string (can you check your code and attach any transformation?), or it may be a problem with the MIME message builder, which, as fare as I understand, is some rmail library (but I could not find it, so can't test against it)