I just modified the sub only to change multipart boundaries. There is the case where binary encoded data are in the body with '\n' char and I don't want to mess with those

my $old_stringify_body = \&MIME::Entity::stringify_body;

*MIME::Entity::stringify_body = sub {
    my( $self ) = @_;
    my $body = &$old_stringify_body;
    $body=~/(----+=_\d+-\d+-\d)/;
    my $boundary=$1;
    $body =~ s!\n$boundary\n!\r\n$boundary\r\n!g;
    return $body
};
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Great thanks Corion for the help!