Trying to debug postfix 'unknown mail transport error'

[Patrick Mahan]

FreeBSD 11.2, Postfix 3.3.2, Dovecot 2.3.4

Random user verification failures are occurring and I am not sure why.

Here's an example -

>From /var/log/maillog:

Failure:

Jan 21 12:20:41 ns postfix/smtpd[31736]: NOQUEUE: reject: RCPT from
mta2.email.famousfootwear.com[136.147.183.86]: 450 4.1.1 <tr...@mahan.org>:
Recipient address rejected: unverified address: unknown mail transport
error; from=<
bounce-299_html-404541337-2436561-7222883-9...@bounce.email.famousfootwear.com>
to=<tr...@mahan.org> proto=ESMTP helo=<mta2.email.famousfootwear.com>
....
Jan 21 12:20:41 ns dovecot: lmtp(31763): Connect from local
Jan 21 12:20:41 ns dovecot: lmtp(31763): Disconnect from local: Client has
quit the connection (state=READY)

Success:

Jan 21 12:20:41 ns postfix/lmtp[31778]: 189C1A2DFB1: to=<tr...@mahan.org>,
relay=ns.mahan.org[private/dovecot-lmtp], delay=0, delays=0/0/0/0,
dsn=2.1.5, status=deliverable (250 2.1.5 OK)

I am not sure why it sometimes fails.  Any help is appreciated.

My config (that I think are related)

Postfix mail transport:

root@ns:/usr/local/etc/dovecot # postconf -n | grep transport
mailbox_transport = lmtp:unix:private/dovecot-lmtp

Dovecot auth config

auth_cache_size = 10 M
auth_username_format = %Ln

service auth-worker {
  user = postfix
}

service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
}

Dovecot lmtp config:

protocols = imap lmtp sieve

service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0666
    user = postfix
  }
}

protocol lmtp {
  mail_fsync = optimized
  mail_plugins = " sieve"
}