Thanks for your answer !
I have Ubuntu 20.04 on Hyper-V (Windows Server 2019) behind a Zyxel USG 310. Like I said, I have this problem when I open mail ports (587, 465, 993, etc.) from the USG 310 to my server. When mailcow on my server can be reached from the internet, DNS doesn’t work anymore, just not for mailcow but Ubuntu entirely (nslookup, web browser, etc.). It’s so strange because it doesn’t happen if mailcow is down during my tests.
I have disabled all firewalls for mailcow just a moment to see, but the problem still here.
Here the network path : mailcow (Ubuntu 20.04 on Hyper-V) –> Windows Server 2019 (Host) –> USG 310 (with just ports fowarding and routing) –> ISP (with same ports fowarding).
Thanks again !