If your Microsoft 365 account was disconnected after connecting, chances are their SMTP is disabled.
You need to 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
Enable SMTP for Outlook accounts directly from Microsoft
Here are the steps to enable SMTP for your Outlook accounts directly from Microsoft:
Login to your admin console and from this link - 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
Note: In case you are getting the error "Your account already exists" when trying to add it, please try to sign in Instantly from the Incognito window.
Enable SMTP for Outlook accounts purchased on Godaddy
If you get Outlook accounts from GoDaddy, you can check out this guide.
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: