Messages without a Date header get a bogus fallback value

This is a problem I noticed with the https://asciinema.org/ login emails. The messages don't have a Date header (but I see `X-Amavis-Alert: BAD HEADER SECTION, Missing required header field: "Date"`) and get a fallback value of UNIX time plus my UTC offset, which puts them at the bottom on my inbox view, and I wouldn't find them without querying for them.