Use valid_email_address()?

It isn't really a validation check so much as a count check. The idea is to check for multiple email addresses, which valid_email_address() doesn't support.

As a complete side note, but on the topic of that function, I have run in to a number of issues where valid_email_address on a number of hosts doesn't behave properly with an otherwise valid address. Usually it is a slightly older (but still supported) version of php causing a problem with the filter format, but it makes predictable portability much harder, so it isn't always an ideal helper function.