Looks to me like the FoxyCart checkout page does not verify that an e-mail address is valid... which leads to occasional badness.
For example, if someone went through the checkout page and entered their e-mail address as john.smith, the order *would* get processed, and the order confirmation e-mail would be sent to address firstname.lastname@example.org-- usually a non-existent address. (You might think this never happens, but we've actually had three cases of it-- that I noticed --in the last year.)
It seems to me that perhaps basic address verification shouldn't be too difficult or resource intensive. Check for an '@'