Messages without a "Date" header should use "Delivery date" as the message's date.
I would recommand that, too.
Please take this into account also for all kinds of filters (incomming, "Verarbeitungsregel", ...).
Filtering a mail by "age" wich does not have a "Date" field in the header causing a "True" for the filter. Mails without a "Date" are extrem old. ;)