Email marked as spam if using temporary address

Have not yet transferred my domain or pointed to Hostpapa site as I want to get any probs ironed out first. It's all working properly now using the temporary site address provided except that test mail sent from PHP mail is all ending up in the deleted items box, marked as Spam by my anti-virus.

Would I be correct in thinking that this is only because I am using the temporary site address (so the from headers don't match origin etc) and that this is a problem that should disappear once I have the domain pointed at the Hostpapa server?

Cheers for any opinions.