First - update your resume.
Second - Explain to him again that the reason folks get 'mailbox not found' messages is because the person/account is no longer there. This is normal and should not be changed - the thing that SHOULD be changed is where customers send email.
Setting up email addresses such as ourIBMcustomer and pointing that to the appropriate group of folks that can answer these emails.
So one of your customers wants to send an email to a person or a group of folks at your company for information. The customer gets an email address from your company such as
ourIBMcustomer@yourdomain.com. In your sendmail server (You did not post the OS - I'll give example of Solaris and Sendmail) in /etc/mail/alias file put the following entry
ourIBMcustomer: myboss, myaccount, personabouttoleave
So any mail coming in for ourIBMcustomer would go to your boss, you, and the person about to leave your company.
Then you run the newaliases command and now any email coming in for ourIBMcustomer goes to those folks. When the person does leave, you remove personabouttoleave from that line, save the alias file, and run the newaliases command again.
Now the email only goes to you and your boss (or you can add another new account). You have to teach your customers that if they want a response from email, they must send to the account set up for their company (or accounts - you can have many).
The headache is keeping up with it - hopefully you know when folks are leaving - another thing you can do is for the person who just left the company is set up an alias to send any of their email to their boss.
john.doe: jane.doe
Any email coming in for john.doe would now go to jane.doe (after the newaliases command is run).
Or a .forward file could be put into the currently gone user's home directory to forward all email to someone else. Locking the account would not effect email deliver but would be needed to insure the ex-employee did not get into your servers via that account.
If you still want to mess around with sending all email to your boss, you would have to edit the sendmail.cf file and redirect email that fails to have a valid receipent to send him the message - a wildcard alias would not work since he would get ALL email - not just invalid email.