MFMailComposeViewController Large Pdfs attachments get stuck

Question

So I am using MFMailComposeViewController to attach pdf files to an email and send it out using the default iOS mail app set up with an exchange account.

If I send a file with size 4863K it works perfectly. Everything goes off as expected.

If I send a file with size 5688K it seems like its working and gets MFMailComposeResultSent as the result sent back to the delegate.

However, the mail never arrives and when I look in the mail app, I see that it is stuck in the Outbox with an error of:

The server rejected the message

What's super weird is that when I try to resent the message from the outbox, it sends out perfectly fine and gives no further error message.

I am sort of at my wits end and have not the first clue what could be going wrong. Clearly I cannot screw around with apple's code, but there don't seem to be many options with the MFMailComposeViewController that I could possibly have screwed up.

The code is fairly straightforward:

- (void) setAttachments:(NSArray*)attatchments
                ofDraft:(MFMailComposeViewController*)draft
{
  if (attatchments)
  {
    for (NSString* path in attatchments)
    {
        NSData* data = [self getDataForAttachmentPath:path];

        NSString* basename = [self getBasenameFromAttachmentPath:path];
        NSString* pathExt  = [basename pathExtension];
        NSString* fileName = [basename pathComponents].lastObject;
        NSString* mimeType = [self getMimeTypeFromFileExtension:pathExt];

        // Couldn't find mimeType, must be some type of binary data
        if (mimeType == nil) mimeType = @"application/octet-stream";

        [draft addAttachmentData:data mimeType:mimeType fileName:fileName];
    }
  }
}

The fact that this works for smaller files implies to me that there is nothing wrong with this code and it is copied out of a cordova plugin that sees pretty wide usage. But for whatever reason, files that are over ~5MB just fail for no apparent reason.

Thanks for your help.

Update I captured the http traffic and discovered that in the situation where the mail is sent using the MFMailComposeViewController, the POST request has a content-length of 0, which is not allowed, so the server returns a 400 error. When I try to resend the same message from the mail app directly, the content-length is the size expected.

This appears to be a limitation of the MessageUI framework, but I cannot verify that there is a universal limitation imposed by Apple.