lmtp on frontend responds with "550 Mailbox unkown" while mupdate unavailable and mailbox not yet in mailboxes.db

[MASHtm]

After starting my fresh 3.8.6 frontends with an empty mailboxes.db and waiting for mupdate to sync it the first time I saw "some"

Aug 23 01:24:18 justin Flmtp[7242]: kick_mupdate: can't connect to target: Connection refused
Aug 23 01:24:18 justin Flmtp[7242]: verify_user(xxxxxxxxx) failed: Mailbox does not exist

messages in the log and was rejected via LMTP with

Aug 23 01:24:18 roger exim[2510260]: 1upakk-0000000AVbG-1QCM ** xxxxxxxx@xxxxxxxxxx <xxxxxxxxxxxx> ...: SMTP error from remote mail server after RCPT TO:<xxxxxxxxxxxx@xxxxxxxxxxxx>: 550-Mailbox unknown.  Either there is no mailbox associated with this\n550-name or you do not have authorization to see it.\n550 5.1.1 User unknown DT=0s

I would have stopped my MTAs upfront, but I honestly didn't expect that behaviour.

I think at least lmtpd should temp reject with "4xx" if it tries to ask mupdate but fails to.