Email-to-Case functionality has been stopped for a while due to some issue in our Org. Inbetween the time the Case was not created in the Salesforce. We received all the emails as errors in our mailbox and wanted to attach them to the relevant Case.
CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY : CaseTrigger: execution of AfterInsert
caused by: System.DmlException: Update failed. First exception on row 0 with id 5004a000008GWGgAAO; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, Cases in queue cannot be modified.: 
Class.CaseTriggerHandler.updateCaseQueue: line 132, column 1 Trigger.CaseTrigger: line 250, column 1
From: email@example.com(Sam yuk) Sent: Wed May 18 04:26:09 GMT 2022 To: [firstname.lastname@example.org] Subject: test case- ASAP assist -