If someone is having their Microsoft 365 account disconnected after connecting, chances are their SMTP is disabled.
Follow these steps to fix the Microsoft/Office 365 disconnection problem:
Enable SMTP and then wait for an hour. Waiting for an hour is a crucial step. This allows Microsoft to propagate the changes and share new tokens with us.
Here's the official Microsoft guide on it: https://aka.ms/smtp_auth_disabled or you can:
Login to your admin console and from this link https://admin.microsoft.com/adminportal/home#/users - select the account
Click Mail in the flyout that appears, and then click Manage email apps
Verify the Authenticated SMTP setting is enabled
Save changes
After one hour, remove accounts from Instantly and then connect them again
Click on your account's three dots in Instantly and choose Remove Account
In case you are getting the error "Your account already exists" when trying to add it, you need to sign in to Instantly from the Incognito window.
Microsoft SMTP (Outlook) purchased on Godaddy
If you have your Outlook account from GoDaddy, you can check out the guide here: https://in.godaddy.com/help/enable-smtp-authentication-40981
For Microsoft accounts to work, It is important to enable SMTP in the Godaddy Admin section, wait for an hour and then add the account to Instantly.
If your accounts keep disconnecting, delete them from Instantly, re-enable SMTP, wait for an hour and connect your accounts again.
Start from here: https://account.godaddy.com/products
This video explains the process: