I think the problem could be in the attachment type that might be blacklisted by receiver's mail system. What type of attachment are you trying to send (.exe, .mpg...)?
On the other hand, you can also try Giant's suggested method, i.e. ask the other receiver to forward the mail to the problem user.
But I'm sending some ordinary format, i.e. excel, tif. We did another experiment today by sending a .doc file to 2 recipients in the same company, 1 rec'd well and the other still couldn't see any attachment. 同 Giant 的情況一樣,連萬字夾都冇。
It looks like the problem is in the recipients' mail system. Also, how big is your attachment? Some mail boxes have limit on the size of attachment to be received.