Using classic Outlook for Windows in business environments
I have fixed this issue after looking at multiple suggestions. In Outlook 2019 Desktop app on Windows, select File->Account Settings->Account Settings-> and highlight the Outlook/Hotmail account and then Repair on the line above the accounts. Select Repair and when the next screen comes up for IMAP Account Settings, select the "Outgoing Mail" line. At the bottom, select "My outgoing (SMTP) server requires authentication" and then select "Log on using user name and password" and enter the correct information for logging on to the email account. Check the box for "Remember password". Select Next and this should fix the issue. It did for me. Hope this helps.