A couple possibilities:
1) The email was spoofed and mailservers are rejecting the virus emails and returning them to your clients email address thinking that's where they came from. The best solution here would be to change email addresses or just set up a filter to automatically delete or mark these messages as spam. If your client owns or operates the mailserver and wants to change email addresses, just have them blackhole any messages coming in for the old user.
2) The email account was compromised and viruses were being sent out and then bounced back. The best solution here would be to change your password and/or get a new account.
3) The mail server was compromised and viruses were being sent out and bounced back. You would have to get in contact with the administrator or service provider.
#1 is most likely the issue. But just to be safe, you can try contacting the email service provider.