Why is deliver.db so large?
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Having a large deliver.db (on the order of tens of megabytes) is not
uncommon, since a record is kept of every message that goes through
lmtpd (so that duplicate suppression can do its work), so there's a
lot of data there.

Note also that it is common for databases to have unused space that
may not be immediately recovered for performance reasons (this is the
case with Cyrus Skiplist) You can control the size by increasing the
frequency at which it gets pruned.
