Notification email trigger for VisitorCenter app

VisitorNotify

trigger VisitorNotify on Visitor__c (before update) {
    List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
    Map<Id, Visitor__c> emps = new Map<Id, Visitor__c>([select Id, User__r.Name, User__r.Email
                                                       from Visitor__c where Id in :Trigger.new]);
    for (Visitor__c v : Trigger.new) {
        Visitor__c oldv = Trigger.oldMap.get(v.Id);
        if (v.Status__c == 'Checkedin' && oldv.Status__c == 'Pending') {
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            Visitor__c emp = emps.get(v.id);
            List<String> sendTo = new List<String>();
            sendTo.add(emp.User__r.Email);
            mail.setToAddresses(sendTo);
            mail.setSenderDisplayName('VisitorCenter System');
            mail.setSubject('Visitor at ' + String.valueOf(v.Date__c));
            String body = 'Hi ' + emp.User__r.Name  + ',<br/>This is to notify you that <b>' +
                v.FirstName__c + ' ' + v.MiddleName__c + ' ' +  v.LastName__c + '</b> has been ' + 
                'approved to visit you at <b>' + String.valueOf(v.Date__c) + '</b>.<br/>Check the ' +
                'VisitorCenter app on your phone for more details.<br/>Please do not reply to ' + 
                'this system-generated mail.';
            mail.setHTMLBody(body);
            mails.add(mail);
        }
    }
    Messaging.sendEmail(mails);
}