Also, what Edition (Cloud, Desktop (Windows, Linux, Mac) or Server (Windows, Linux, Mac)), and which version (should be same as listed on https://www.manager.io/download) If the version is not up to date, make a backup of each business and install the update. Cloud edition is updated by Manager.
Also try to create a test business and only enable some basic tabs and enter the smtp information and try to send an email. You can also try to send an email using: