Act! Premium Version 21.0.295.0, Update 4 on Windows 10 Pro (64-bit) Pending Smart Task Steps fail to clear after completed.
Schedule > Run Smart Tasks, Run a Task with Send Mail step. I'm running Outlook Office 365 MSO (16.0.11029.20045) 32-bit version.
The Pending Smart Task Steps window opens. Act! sends email message to Outlook. However, the Send E-mail task still appears on the Pending Smart Task Steps window. If you "Run" the Send E-mail task, a duplicate email is sent to Outlook and then the Send E-mail task clears from Pending Smart Task Steps list. If you Preview task, an Outlook compose window appears with the smart task message, after which, the Send E-mail task clears from Pending Smart Task Steps list.
This has been an ongoing issue and needs to be addressed in the bug fixes.