How to write test class for outbound email services

what do you actually want to test? the execution of the code or that the message gets delivered?

how about you delegate it to a separate method sendNotication() which you can test independently. furthermore you can test it via console as well

batch class

global class MyBatch implements Database.Batchable<sObject> {

// BATCH CONTRACT ...

global void finish(Database.BatchableContext bc){

if (MY_CONDITION)
{
sendNotification();
}
}

public void sendNotification(){

Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
// CONFIGURE MAIL

Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
}

}

test class

@isTest
private class MyBatchTest {
static testMethod void sendNotificationTest(){

Test.startTest();
Integer emailbefore = Limits.getEmailInvocations();
MyBatch batch = new MyBatch();
MyBatch.sendNotication();

system.assertNotEquals(emailbefore,Limits.getEmailInvocations(),'should have decreased');
Test.stopTest();
}
}