FileMessage.m

- (id)MAParsedMessage {
    // This method is called, when a message is opened from outside the library (an .eml is loaded.)
    id messageData = [((MCMessage *)self) messageDataIncludingFromSpace:0x0 newDocumentID:0x0 fetchIfNotAvailable:0x1];
    id parsedMessage = nil;
    if (messageData) {
        MCMimePart *topLevelPart = [[MCMimePart alloc] initWithEncodedData:messageData];
        MCMimeBody *body = [MCMimeBody new];
        [body setIvar:kMimeBodyMessageKey value:self];
        [topLevelPart setIvar:kMimePartAllowPGPProcessingKey value:@(YES)];
        [topLevelPart setIvar:@"MimeBody" value:body];
        [body setTopLevelPart:topLevelPart];
        [(MCMimePart *)topLevelPart parse];
        parsedMessage = [body parsedMessage];
    }
    return parsedMessage;
}

Original.asm

void * -[MCMessage parsedMessage](void * self, void * _cmd) {
    r12 = 0x0;
    r14 = [[self messageDataIncludingFromSpace:0x0 newDocumentID:0x0 fetchIfNotAvailable:0x1] retain];
    if (r14 != 0x0) {
            r15 = [[MCMimePart alloc] initWithEncodedData:r14];
            rbx = [MCMimeBody new];
            [rbx setTopLevelPart:r15];
            [r15 parse];
            r12 = [[rbx parsedMessage] retain];
            [rbx release];
            [r15 release];
    }
    [r14 release];
    rax = [r12 autorelease];
    return rax;
}