It finally appeared to stop yesterday after I made below changes to ConfigServer (Firewall Configuration):
PT_USERMEM = 1024 (Default 256)
PT_USERTIME = 0 (Default 1800)
Granted, this may happen again, but I did not receive single notification of "Excessive memory usage" notification overnight since yesterday. It was not mailman, but I later found out it was caused by Imunify360. I had tracked down to WebShield in Imunify360 that was causing a lot of memory usage, and actually it was causing hourly notification over the weekend (pointing to /usr/sbin/imunify360-webshield), but once I made above change to ConfigServer, it stopped finally.
Thank you for your support.