I have found the answer to this, and it is annoyingly/frustratingly simple! Basically the reply to addresses needed to be added before the from address as such:
$mail->addReplyTo('[email protected]', 'Reply to name');
$mail->SetFrom('[email protected]', 'Mailbox name');
Looking at the phpmailer code in more detail this is the offending line:
public function SetFrom($address, $name = '',$auto=1) {
$address = trim($address);
$name = trim(preg_replace('/[\r\n]+/', '', $name)); //Strip breaks and trim
if (!self::ValidateAddress($address)) {
$this->SetError($this->Lang('invalid_address').': '. $address);
if ($this->exceptions) {
throw new phpmailerException($this->Lang('invalid_address').': '.$address);
}
echo $this->Lang('invalid_address').': '.$address;
return false;
}
$this->From = $address;
$this->FromName = $name;
if ($auto) {
if (empty($this->ReplyTo)) {
$this->AddAnAddress('ReplyTo', $address, $name);
}
if (empty($this->Sender)) {
$this->Sender = $address;
}
}
return true;
}
Specifically this line:
if (empty($this->ReplyTo)) {
$this->AddAnAddress('ReplyTo', $address, $name);
}
Thanks for your help everyone!