/usr/sbin/sendmail: unexpected response 550 to RCPT TO command
/usr/sbin/sendmail: unexpected response 503 to DATA command
[Fri 2 Jun 2017 04:24] [CL] 1 failed (will retry later)
To find the email, check the eventlog table in MySQL by sorting last ID first:
Error sending email to example@example.com Could not instantiate mail function.
UPDATE 5/26/22
Same business for phplist v3.6.5
[Wed 25 May 2022 12:00] [CL] 8 failed (will retry later)