When scanning some messages which themselves have attached email messages, Messaging Gateway (SMG) does not render an expected content filtering verdict on files which are included in the attached email message file i.e. an email message attachment which contains other file attachments. SMG does not render an unscannable verdict on the message but also does not identify the nested file attachments as expected.
Decomposition of some proprietary message file formats does not produce a list of identifiable attachments for content filtering. Essentially, the attached email message file can be deconstructed but the parts are not identifiable as standard MIME attachments.
This is a known issue with some forms of proprietary email file formats and does not affect standard RFC822 attachments. This issue will be addressed in a future release.
Depending on the attachment type, this issue may be addressed by unchecking "Extract OLE 1.0 native only" in the Protocols >> Settings >> Content Scanning >> Advanced page.