I use digital ocean myself (over 3 years with little issues except for the extortionist practices of UCEPROTECTL3 - so I don’t send mail to outlook.com places). But in when I just updated MIAB I noticed that in the status it gives me the error:
“Outbound mail (SMTP port 25) seems to be blocked by your network. You will not be able to send any mail. Many residential networks block port 25 to prevent hijacked machines from being able to send spam. A quick connection test to Google’s mail server on port 25 failed.”
I think this is caused by the check which first used ipv6. I modified /etc/main/postfix.cf and changed
inet_protocols = all
to
inet_protocols = ipv4
and did a service restart postfix, and then from the status I get no error anymore:
“Outbound mail (SMTP port 25) is not blocked.”
If you check the syslog, I found the following which led me to the changing that parameter:
postfix/smtp[11173]: connect to gmail-smtp-in.l.google.com[2607:f8b0:4004:c06::1b]:25: Network is unreachable
Not sure if this helps but - at least if cleans up my status in the admin portal.