If you have an account hosted by Microsoft, such as Hotmail, Outlook, Office, or Live.com, and Postbox is taking a long time to load messages (showing you a spinner), this is because Microsoft recently changed the name of their incoming server to outlook.office365.com.
To fix this, update your incoming server name in account settings.
macOS
- Go to Preferences > Accounts
- Select the account that is experiencing the problem
- Set Incoming Server to: outlook.office365.com
- Close the Preferences window
Windows
- Go to Tools > Options > Accounts
- Select the account that is experiencing the problem
- Set Incoming Server to: outlook.office365.com
- Click OK