You have 2 options to mass delete unresolved tasks, we'll go through both options.
Mass Delete Records
1. Click Setup.
NOTE: You can only delete up to 250 records at a time using this wizard.
2. Under "Administer," click Data Management | Mass Delete Records.
3. Click Mass Delete Activities.
4.. Setup your criteria to include the filter: Subject STARTS WITH "Unresolved Email:" You can also use other filters to narrow down your search results.
Create a report and use Delete function of Data Loader
NOTE: Archived Activities aren't listed in Reports. Use the "Export All" function in Data Loader to export Archived Activities and then perform a Delete.
1. Click the Reports tab.
2. Click New Report.
3. Click Activities folder | click Task and Events.
4. Click Create.
5. Filter your Data. Make sure that the filter Subject starts with "Unresolved Email:" is included and that the Activity ID is added on your report.
6. Run Report and click on Export Details to export the report.
7. Use Data Loader to Delete the records.
Deleting Multiple Records and Reports
Performing Mass Deletes