Scenario
T2T Migration + AD (Source) -->AD (Target) Migration Using QMM
Source = Hybrid O365 / Exchange
Target = AD with Exchange Management Only (No Hybrid)
Issue
After successful Migration of Mail data, Setting the the Delegates fail with erro Cannot set delegate Failed to add one or more delegates. All objects Matched ok
Problem
This is caused if the source domain is in "Hybrid" in which the "publicdelegates" attributes get written back to the On-Prem AD, In turn this is migrated to the Target AD, That is NOT in hybrid mode.
This then stops the public delegates getting written in O365 as its populated with data from on-prem AD.
Resolution
Clear the "Publicdelegates" attribute and exclude from QMM DirSync. and re-run