Using Outlook 365 which works fine when connected via local router, but not when using the mobile phone as a hot spot.
Actually I have two routers, 1) Post Office via broadband, 2) router with data only SIM card (EE) . . both work fine
But when connected to my iPhone 12 (also with EE) as a hot spot, I get the above error.
Strangely, using my iPhone X (with Three) is also fine
Sorry to hear of the issues that you've been having with your emails. Is this still happening to you as it has been over a week since you last posted?
Do you have any content blockers over your mobile account that would stop messages being sent?