- Update oversigned (frozen) header field list to reduce signature
fragility (removes 'date' and 'subject' fields from being oversigned by
default - see usage section of README for information on how to restore
the previous behavior)
- Added new add_should_not for DKIM/ARC classes to prevent additional
header fields from being signed
- Added 'from' to should sign list (to prevent it from not being signed at
all in the unusual event that 'from' is locally removed from the frozen
header field set (LP: #1525048)
- Updates for experimental ARC support:
- Specified that for ARC, Authentication-Results should not be signed