Many times, I've sent emails to old company contacts and gotten no reply, only to eventually learn they no longer work there. Don’t waste your clients' time waiting to hear back; set mail forwarding rules on your server so that a current employee else gets the message and can reply.
The process differs depending on your mail host, but most small businesses will have access to an online control panel. Log in there, and open the mail forwarding section. Choose the option to add a forwarding rule, and enter the old worker's address. Enter the destination address for you or a current employee. Save the results.
Within Outlook on your PC, create a rule to store these forwarded messages in their own folder. Choose File, New, Folder, and name it related to the old employee. In Tools, Rules and Alerts, click New Rule. Pick Check messages when they arrive, and click Next. Click the checkbox for with specific words in the recipient's address, then click the highlighted specific words in Step 2. Enter the old employee's full address. Click Add, click OK, and click Next. Choose move it to the specified folder. Click specified in Step 2. Pick the new folder, click OK, and click Finish. Click Apply.
I prefer this setup so a new employee can introduce themselves to the client and also keep that mail separated. However, you can also add an auto-responder to immediately reply to the sender with the new contact info. Just visit the auto-responder area at the host's control panel, enter the old employee's address, enter your new address in the From field, and create the rest of the message.