It could be. I know I’ve run into that when trying to re-connect to my Onedrive account – I see an explicit “can’t connect to 127.0.0.1” error on the sign-in webpage. I don’t get the web error page when trying to log in to 365, but something other than the browser might be getting blocked, too. I hope I don’t need to fool around blindly with my hosts file trying to find what’s wrong.
BTW, onedrive and outlook mail access were both working sometime around August/September-ish, and I definitely haven’t changed anything in defender since then since it’s been broken for quite a while (since sailjail?).
edit:
Looks like all the defender hosts rules disappeared along with working defender UI sometime while phone was lying in a drawer waiting for volte and occasionally getting update but little use. So nothing seems to be getting re-routed to 127.0.0.1 or 0.0.0.0 anymore. Guess I can’t blame that, then.
But pinging www.microsoft.com and www.outlook.microsoft.com seems to show them being forwarded/cached/whatever by akamai, so that issue is looking more guilty by the minute.
edit 2:
I get the same error trying to log in to the 365 outlook mail with POP3 and also webmail interface. Sure looking more like Sailfish browser trouble.