A live mask accepts mail at RCPT TO and delivers to its owner, with an
X-Masked-Email header naming it. A disabled mask files straight to Trash,
past the owner's Sieve script. Deleted and expired masks are refused like
unknown addresses, without being cached as unknown. Mail moves lastMessageAt
and turns a pending mask enabled. Sub-addresses on a mask work.
Subspace _ holds the fork's own data, with its own SQL table and RocksDB
column family, and is part of backup. The masked_email module keeps each
mask's state, last mail and pending deadline beside upstream's record, an
index from address to mask with tombstones, and a per-account change log;
and generates addresses in the fork's format.