Hello,
I have found this statement in setup guide:
Firewall settings
If your machine is behind a hardware firewall (or virtual equivalent, such as an AWS security group), ensure that the following ports are open: 22 (SSH), 25 (SMTP), 53 (DNS; must be open for both tcp & udp), 80 (HTTP), 443 (HTTPS), 465 (SMTP submission), 993 (IMAP), 995 (POP) and 4190 (Sieve). It doesn’t hurt to block other ports, but your box will take care of that itself by configuring a software firewall on the machine itself.
Which software firewall is configured on MiaB?