|550 5.7.1 is generally a content filter rejection program. Likely that message is being delivered to your Exchange server and Exchange is making its own message that your user is getting.|
I'd check mxtoolbox.com to make sure your records are correct, a lot of companies are cracking down on making sure mail delivered has proper reverse DNS entries, etc. If the original message has specific items that would score against a content filtering system (no message body, no subject, attachments with suspicious extensions, etc.)
Without more details, it's impossible to tell what's CAUSING the problem, but troubleshooting it should start with making sure your DNS and MX records are properly setup and then test different messages to the same e-mail address that are properly formatted and not suspicious.
Otherwise, if you work with that company often, you can call and ask their network administrator to whitelist your domain.