There seems to be an issue with the contact form and Yahoo addresses, potentially others too. The progression goes like this:
User contacts us via the Contact page.
We reply within 24 hours (if possible)
User does not receive email reply. Uses contact form again.
We reply again.... message is not received.... and so on.
Several possibilities here. I suspect Yahoo is flagging my replies as spam, since they are a standard format and appear (to Yahoo) unsolicited. Some spam is not even delivered to spam folders, nor is it bounced. I cannot confirm that this is happening without some test cases.
There are two ways to try and fix this.