Here is a tough one to explain but I'll give it a shot and hope there is someone who had this problem and may be an expert on firewalls and exchange. Our IT department at work (which reports to me ironically) can't resolve it. Two days ago in the middle of the day, Mail had an error message next to it. I clicked it and it asked me for my exchange password. I have been getting my work email through Mail for 6 months with no issues. I enter my password and I get "failed to verify username or password". Nothing changed on my Mac or the exchange server environment (that I am aware of). IT tells me the server is authenticating the ask of Mail but my Mac disconnects immediately. Here are things I know: - same issue with using Outlook 2016 - same issue trying to get mail for a different exchange account through my machine. It's not just my account. - Outlook web mail works fine - I booted to an old build of High Sierra when it worked fine and have the same issue - Does this whether on the company ethernet or home on my wifi - Phones, iPads, Win10 machine all getting email just fine That's about all I can tell you. Just a shot in the dark that this may ring a bell based on the limited information I can provide. My first thought when it started was that the firewall was blocking my MBP. Thanks in advance.