Hello,
I have the below async class  and its test.
What I failing with is how to cover test percentage for Messaging.SingleEmailMessage Invocation when callout fails..
I have already gone through multiple links on stack exchange and Apex forum, tried some approaches but none worked for me.
Can some one please assist in what I may be doing wrong here.
Code coverage under SUCCESS is 78% while under FAILURE is 24%.
 

public class DeleteExpertInSelene {
	//method to be invoked by ProcessBuilder apex
    @InvocableMethod
    public static void deleteAccountInSelene(List<Id> acctIds){
        Account acc = [SELECT Selene_ID__c FROM Account WHERE Id = :acctIds[0]];   
        System.enqueueJob(new QueueableSeleneCall(acc.Selene_ID__c, acc.Id));
    }
    
    // future method to make delete experts
    @Future(callout=true)
    private static void deleteToSelene(Decimal userId, Id acctId) {				 
        List<String> EMAIL_RESULTS = new List<String>{System.Label.Email_Selene_List};
        List<String> EMAIL_RESULTS_2 = new List<String>{System.Label.Email_Selene_List_2};
        String tokenVar = '';
        tokenVar = 'Token '+GetSeleneID.getAuthorizationToken();
        HTTPRequest req = new HTTPRequest();
        req.setEndPoint('some api'+ userId);     
        req.setMethod('DELETE');
        req.setHeader('authorization',tokenVar);
        req.setHeader('Accept','application/json;charset=UTF-8');
        req.setHeader('Content-Type', 'application/json;charset=UTF-8'); 
        req.setHeader('Cache-Control', 'no-cache'); 
        req.setHeader('x-api-key', 'some key');
        req.setTimeout(120000);                                         
        HTTP http = new HTTP();
        HTTPResponse res = http.send(req);
        
        if(res.getStatusCode() != 200){
            System.debug('Failure: ' + res.getStatusCode() + ' ' + res.getStatus());
            // Send email
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            mail.setToAddresses(EMAIL_RESULTS);
            mail.setCcAddresses(EMAIL_RESULTS_2);
            mail.setSenderDisplayName('Salesforce Administrator');
            mail.setSubject('Delete To Selene Failed ');
            mail.setPlainTextBody('some mssg');
            Messaging.sendEmail(new List<Messaging.SingleEmailMessage>{mail});
        } else {
            if(res.getStatusCode() == 200){
                System.debug('Success: ' + res.getBody() + ' ' + res.getStatus());
        }
        
    }
           
 }   
    //queueable class to enque the put request
    class QueueableSeleneCall implements System.Queueable, Database.AllowsCallouts {
        private Decimal seleneId;
        private String Id;
        public QueueableSeleneCall(Decimal userId, Id acctId){             
            this.seleneId = userId;
            this.Id = acctId;
        }
        public void execute(QueueableContext context) {
            deleteToSelene(seleneId, Id);                               
        }
    }
}

Test Class...

@isTest
public class DeleteExpertInSelene_Test {
	  @testSetup static void testSetupdata(){
        //create the account record
        Account acc1 = new Account();
        acc1.Name = 'ABC Corp1';
        acc1.Email__c = 'test@testmail.com';
        acc1.Phone = '5554446767';
        acc1.Legacy_GID__c = '54321';
        acc1.Display_Name__c = 'abc1';
        acc1.Status__c = 'Live';
        acc1.Selene_ID__c = 112233445.00;
        insert acc1;
        //create the account record
        Account acc2 = new Account();
        acc2.Name = 'ABC Corp2';
        acc2.Email__c = 'testmail@test.com';
        acc2.Phone = '6665554343';
        acc2.Legacy_GID__c = '12345';
        acc2.Display_Name__c = 'abc2';
        acc2.Status__c = 'Live';
        acc2.Selene_ID__c = 112233446.00;
        insert acc2;
    
  }
  
       
  @isTest static void testPostCalloutSuccess() {
      Account acc = [Select Id, Name FROM Account WHERE Name = 'ABC Corp1' Limit 1];
      List<Id> accList = new List<Id>();
      accList.add(acc.Id);
      System.assertEquals('ABC Corp1', acc.Name);
      System.assertEquals(1,accList.size());
      // Set mock callout class 
      Test.setMock(HttpCalloutMock.class, new SeleneCalloutMockService()); 
      // This causes a fake response to be sent
      // from the class that implements HttpCalloutMock. 
      
      Test.startTest();
      DeleteExpertInSelene.deleteAccountInSelene(accList);
      //Integer invocations = Limits.getEmailInvocations();
      Test.stopTest(); 
      
      
      // Verify that the response received contains fake values        
      acc = [select Selene_ID__c from Account where id =: acc.Id];
      System.assertEquals(112233445.00,acc.Selene_ID__c);
      
    }
    
    @isTest static void testPostCalloutFailure() {
        Account acc = [Select Id, Name FROM Account WHERE Name = 'ABC Corp2' Limit 1];
        List<Id> accList = new List<Id>();
        accList.add(acc.Id);
        System.assertEquals('ABC Corp2', acc.Name);
        System.assertEquals(1,accList.size());  
        
        // Set mock callout class
        Test.setMock(HttpCalloutMock.class, new SeleneCalloutMockService()); 
        // This causes a fake response to be sent
        // from the class that implements HttpCalloutMock. 
        Test.startTest();
        DeleteExpertInSelene.deleteAccountInSelene(accList);
        Integer invocations = Limits.getEmailInvocations();
        Test.stopTest();        
        // Verify that the response received contains fake values        
        acc = [select Selene_ID__c from Account where id =: acc.Id];
        System.assertEquals(112233446.00,acc.Selene_ID__c);
        System.assertEquals(1, invocations, 'An email should be sent');
    }  
    
}

Any help is appreciated.
Thanks!


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