Maybe test further at some other time, ... but VERP disabled
again for now:
/etc/mailman/mm_cfg.py:
# 2019-01-12 Tried enabling VERP ... then disabled it again,
# with the VERP enabled, postings make it to the archive, but don't seem
# to be getting successfully sent?
## Set this Yes to activate VERP probe for disabling by bounce
## VERP_PROBES = No
#VERP_PASSWORD_REMINDERS = Yes
#VERP_PERSONALIZED_DELIVERIES = Yes
#VERP_DELIVERY_INTERVAL = 1
#VERP_CONFIRMATIONS = Yes