Microsoft Outlook must be configured to access Microsoft Exchange without typing in your username and password. The reason this matters is that Epicenter Server runs unattended under Task Scheduler, so it must be able to start Outlook without prompting for a password. Note that leaving Outlook open will work for Send Now, but won't work when Epicenter Server runs a scheduled task.
This is generally not a problem if you are on a domain with a dedicated Exchange Server because everything "just works." However, problems can arise when you use Hosted Exchange Server because Exchange requires a username/password that's separate from your workstation login. Passwords are automatically saved by Outlook 2013/2016/2019/365, but you must make sure you check the box for "Save Password".
Modern Authentication in Microsoft 365 adds complexity. In this case, you must use Outlook 2013 to force a fallback to Basic Authentication because Modern Authentication does not work with Epicenter Server.