[exim] 'Could not complete sender verify'

    When arthurbanana@??? sends a message to
fred@123.456.789.012 exim undelivers the message and logs:

    2021-11-23 04:41:56.239 [23354] H=mail-oi1-f171.google.com [209.85.167.171]:47047 I=[123.456.789.012]:25 sender verify defer for <arthurbanana@???>: host lookup did not complete
    2021-11-23 04:41:56.239 [23354] H=mail-oi1-f171.google.com [209.85.167.171]:47047 I=[123.456.789.012]:25 X=TLS1.2:ECDHE-RSA-AES128-GCM-SHA256:128 CV=no F=<arthurbanana@???> temporarily rejected RCPT <fred@123.456.789.012>: Could not complete sender verify
    2021-11-23 04:41:56.272 [23354] H=mail-oi1-f171.google.com [209.85.167.171]:47047 I=[123.456.789.012]:25 incomplete transaction (QUIT) from <arthurbanana@???>
    2021-11-23 04:41:56.273 [23354] SMTP connection from mail-oi1-f171.google.com [209.85.167.171]:47047 I=[123.456.789.012]:25 closed by QUIT

    What does this mean?  Does it mean that exim (which serves
123.456.789.012) can't verify arthurbanana@????