CiviMail: replies to deleted mailings and from deleted contacts are not being handled correctly

when replying to a deleted mailing or from a deleted contact's email address the reply is not forwarded anywhere and no error is returned during the soap call.

there should either be an error returned during the soap call or the reply should be forwarded to some default email address such as the default From address.

this was reproduced using the soap.php as the return channel on Joomla 1.5.6 using the CiviSMTP service.

to reproduce:

1. send a mailing with "Forward Replies" enabled and "Override VERP address?" disabled to a test contact.
2. reply from that test contact to ensure that replies are being processed and forwarded appropriately.
3. delete the mailing or the test contact
4. reply again from the test contact email address. the reply doesn't get forwarded and no error is returned during the soap call.