Feature request: ignore expired signatures

[sersorrel]

Some DKIM signatures have relatively short expiry periods (e.g. expiry timestamp 24 hours or less beyond the signature timestamp). If I don't open Thunderbird for a day, this results in a lot of false-positive DKIM warnings.

Would you consider adding a feature to improve this situation? e.g. an option to:

• ignore expired signatures entirely (like the existing advanced settings for weak keys/sha1/...)
• check signatures based on the time the mail was received at the server, if known (e.g. Received headers, Delivery-date header, ...)