trigger identifyLastEmailMessage on EmailMessage (after insert, after update) {
    List<EmailMessage> emailThatStartedTrigger = new List<EmailMessage>();
    emailThatStartedTrigger = Trigger.new;
    system.debug('Case Id: ' + emailThatStartedTrigger[0].ParentId);
    List<EmailMessage> allEmailsRelatedToThisCase = new List<EmailMessage>();
    allEmailsRelatedToThisCase = [select Id, Last_Email_for_Case__c, CreatedDate from EmailMessage where ParentId = :emailThatStartedTrigger[0].ParentId ORDER BY CreatedDate ASC];

    if (Trigger.isAfter) {

        for (integer i =0; i< allEmailsRelatedToThisCase.size(); i++) {
            if (i == 0) {
                allEmailsRelatedToThisCase[i].Last_Email_for_Case__c = true;
            }
            else {
                allEmailsRelatedToThisCase[i].Last_Email_for_Case__c = false;
            }

        }

    }
}