Posting how I solved this here as nothing in this forum worked for me:
Instead of adding and Outlook.com or Office365 account with the Automatic Setup, enter account details manually (selecting EMAIL->Other), I used these settings:
- Incoming server:
outlook.office365.com
- Outgoing server:
smtp-mail.outlook.com
- User:
- Password: (Application password generated in your Microsoft Account)
The app password is the important part, the automatic way to add a Outlook account uses a different authentication method which seems to be unstable with eMClient, while with this method eMClient authenticates with this one off app password.
I have not had the error again since.