Это выглядит не так
$request->Items->ForwardedItem->ToRecipients = new MessageType();
$request->Items->ForwardedItem->ToRecipients->Mailbox = new EmailAddressType();
$request->Items->ForwardedItem->ToRecipients->Mailbox->MailboxName = 'Foo Bar';
$request->Items->ForwardedItem->ToRecipients->Mailbox->MailboxAddress = 'Foo@Bar.com';
ToRecipients должен быть массивом типов получателей, поэтому я думаю, что это должно быть
$request->Items->ForwardedItem->ToRecipients = new ArrayOfRecipientsType();
$recipient = new EmailAddressType();
$recipient->Name = 'Homer Simpson';
$recipient->EmailAddress = 'hsimpson@example.com';
$request->Items->ForwardedItem->ToRecipients->Mailbox[] = $recipient;