Leap-mx

o Add PGP key lookup on access check server and reject mail if no PGP key
was found for the user. Closes 6795.
o Fix bounce message recipient. Closes 6854.
o Implement bouncing as per RFCs 6522, 3834 and 3464. Closes 6858.
o Correctly return async bouncer deferred.

o Process unprocessed mail when MX starts (closes 2591).
o Log to syslog. Closes: 6307
o MX log and retry when delivery directories don't exist. Closes 6687.

o Bounce mails when there's a problematic situation for an email,
such as no public key. Fixes 4803.
o Properly log tracebacks for exceptions occuring in the mail
processing loop.
o Add errdecr key defaulting to each mail Soledad Document. Closes
6072.
o Any inconsistent user would make the mx stop decrypting mails for
all subsequent users.

o Use CouchDocument to comply with new Soledad couch backend. Fixes
4475.
o Some emails are multipart and each part has its own encoding.

o Add X-Leap-Provenance header. Closes 4356.
o Add tester script to ease testing problematic emails offline.

o Some mail may be skipped at processing because of possible
problems (like connectivity issues to our couch nodes), MX now
looks for unprocessed mails every half hour and tries to process
them. Fixes 3628.