MIAB is lying, port 25 inbound IS open

[Unsupported modifications due to using Power-MIAB for an SMTP relay]
The MIAB status page is telling me inbound email on my network is blocked. That’s not right, though. I can receive email fine from Gmail and Protonmail, so I have to doubt it.
Anyone know what’s going on? I tried disabling ipv6 (since I don’t get an ipv6 IP, internally/externally), and that fixed nothing.

I believe that MIAB only reports whether port 25 is open outbound, i.e. for sending not receiving. Is that what you are seeing?

Nope. At least, the fork I use reports whether inbound is open or not.

Are you running behind a domestic router/firewall? If so, you’ll need to be sure that outgoing packets addressed to your server’s external address do actually get turned around and seen by the server. (This is how the status tests work.) Some routers call this “hairpin routing” and you might need to enable it. Also fail2ban might be occasionally triggered by hairpin traffic, so add your router’s internal (LAN) address to the "ignoreip = " line of /etc/fail2ban/jail.d/mailinabox.conf.

Time to experiment: Can you connect (use nc) to the external addresses from your server? Can you connect from a remote computer? Try ports 25 (email sending) and 587 (email receipt).

I use the Oracle Cloud for hosting my box, and Sendinblue as an SMTP relay. How would I check it’s allowed that way?

On the subject of “can I connect…”
Port 25:
# telnet box.erisws.com 25 Connection to box.erisws.com:25 - ok 220 box.erisws.com ESMTP Power Mail-in-a-Box (Postfix)

Port 587:
Connected to box.erisws.com. Escape character is '^]'. 220 box.erisws.com ESMTP Power Mail-in-a-Box (Postfix)

From where did you perform those connection checks. Try from an external (remote) computer and also from the box itself. And for testing, it might be clearer to use the actual IP address, so you known what’s happening.

As you’re using Power-MIAB, you might be better talking to them.