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