RFC Compliant emails
Email proxy servers will reject messages that do not comply to the standard for internal mail. If the attachments do not follow the exact structure for that file type, these are also considered suspicious and hence quarantined, just in case they pose any form of threat.
Aspects such as the SMTP headers, message senders, message recipients, message content, and attachments are checked for correct formatting and RFC compliance. An example would be, does the sender domain contain more than one dot in a row such as; mail from: external-domain..com
You can also read the full RFC 821 document for RFC compliant email requirements.
Wikipedia's guide to RFC