If you actually meant your client is using Zencart version 1.3.6, that's the problem. As DrByte said in a thread about the 2006 error:

"Simple: You're using an old version of Zen Cart which sometimes encounters database disconnections and has no recovery code built-in. Upgrading to the latest version would be a good starting point."

For that and security reasons, the client needs to upgrade ASAP. If she's using Paypal Standard, a switch to Express should solve the email issiue.