If your Microsoft 365 account disconnected after connecting, it could be because SMTP is disabled.
Here are the steps on how to fix the disconnected Microsoft/Office 365 accounts.
Login to your Admin Console and select the account.
Click Mail in the flyout that appears and click 'Manage email apps.'
Check the checkbox of Authenticated SMTP to enable the SMTP. You may use their guide.
Click Save.
After an hour, remove accounts from Instantly and connect them again. Waiting for an hour is a crucial step. This allows Microsoft to propagate the changes and share new tokens with us.
Click on your selected account's three dots in Instantly and select Remove account.
Note: In case you are getting the error 'Your account already exists' when trying to add it, you need to sign in Instantly from the Incognito window.
To watch the full video for enabling SMTP for Microsoft Accounts, you may click here.
Microsoft SMTP (Outlook) purchased on GoDaddy
If you have your Outlook account from GoDaddy, you can check this guide here.
For Microsoft account to work, here are the following steps to do.
Enable your SMTP in the GoDaddy Admin Section and wait for an hour.
After an hour, remove accounts from Instantly and connect them again.
However, if your account keeps disconnecting, you may do this.
Delete them from Instantly.
Re-enable your SMTP and wait for an hour.
Connect your account again. You may start here.
To watch the full details about enabling SMTP for Microsoft accounts purchased through GoDaddy, click here.