Email attachments being blocked

[Aziz]

Okay guys, google is being tough on this one. On of the employees here at my work is sending out emails with attachments (dwg files). However, often he gets a complaint back that the attachment wasn't recieved (but the email was). Other times, it works fine. And if he forwards the original email to someone else in the office, and they send it, it receives fine. Our mail server is being blacklisted, and I've sat with him while doing the attachments so I know he's doing it right. The only thing I can think of is that Outlook is using HTML emails and they are plain text, but everyone else in the office is HTML. (Outlook 2003, btw).

So, I come to my family for some pointers. Any ideas? I'm at a loss here.