If your users complain that mail attachments are corrupted, check your antivirus software's configuration first. Have you made any changes or updates to it lately? Antivirus software updates sometimes go out more than once a week. Check with your vendor to see if any problems exist with the latest updates and what to do about them.
You can also turn off your antivirus scanning to see if the corruption problems go away. However, keep in mind that by doing so you become vulnerable to incoming viruses. This potential problem is mitigated somewhat if you have multitiered virus protection. For example, you can scan attachments on the server while turning off SMTP scanning at the gateway.
Application-layer firewalls add security over and above packet filters and stateful inspection, but they can also potentially corrupt attachments. Again, find out if any changes have been made to the configuration or if any patches or upgrades were applied.
If you suspect your firewall is the corruption culprit, you might be able to temporarily circumvent the application-layer filtering by directing SMTP traffic directly to and from the Exchange server. Alternatively, if you're not the firewall person for your organization, contact the responsible department or vendor, and communicate your issues.
While your users may think the Exchange server corrupted their attachments, look at your third-party applications that deal with attachments. Most of the time, you'll find the culprit faster.