I upgraded Antivirus to v20.1.3.22 from an operational configuration using Thunderbird 17.0.6 on Multihome NIC.
After the upgrade, no more POP not SMTP messages could be received or sent. When disabling the Outgoing control from Antivirus, outgoing message could be sent again.
My configuration was:
IP Address #1 192.168.0.26/255.255.255.0
IP Address #2 192.168.1.26/255.255.255.0
GW 192.168.0.1
After investigating with Wireshark, I found that the IP address used wasn't routable i.e. IP Address #2 was used.
When removing IP Address#2, everything is fine again.