Finally removed #IPv6 #AAAA for my mail server so #Thunderbird uses IPv4 only. This IPv6 problem of Thunderbird really kills me.
Even I can access Sogo/Dovecot aso. from the same Workstation at the same time Thunderbird access it, Thunderbird hangs or is brutal slow.
In this case it is LAN to DMZ local without Internet, routed through pfSense. So no Internet, Routing, Tunnel, MTU or other stuff involved.