IMBReader is not enabled by default
According to my observations IMBReader is not instantiated by default in the MultiFormatReader.cs code, therefore Intelligent Mail barcodes are not detected. I know that it is possible to overcome this problem explicitly assigning DecodingOptions to BarcodeReader and used this technique in the Sharp IMG Viewer project:
options.PossibleFormats = new BarcodeFormat[] { BarcodeFormat.All_1D, BarcodeFormat.QR_CODE, BarcodeFormat.DATA_MATRIX, BarcodeFormat.AZTEC, BarcodeFormat.PDF_417, BarcodeFormat.MAXICODE, BarcodeFormat.IMB };
Howerer, I wonder if it is possible to modify your source code to include the IMBReader decoder implicitly when (readers == null || readers.Count == 0)?
Maybe I'm wrong but I think it is disabled because it leads to false-positives. In my opinion, it would be better enabled explicitly.