mail attachment problem

We have 2 Mac Pros and need to access the same email account to download customer files.
The first Mac is running OSX 10.6.8 with Apple Mail 4.5 and the 2nd running OSX 10.5.8 with Apple Mail 3.6.
On the Mail program 4.5 an Illustrator attachment is received but appears with a dashed box around it and when downloaded appears to be corrupt, with a file size of 0 and is not recognizable as an Illustrator file.
The same email attachment is fine on the Mac with Mail program 3.6.
Is there something in the mail 4.5 settings that could cause this?
Thanks!