HI Folks, 
Getting Debuglog Error, 

System.QueryException: Non-selective query against large object type (more than 200000 rows). Consider an indexed filter or contact salesforce.com about custom indexing. Even if a field is indexed a filter might still not be selective when: 1. The filter value includes null (for instance binding with a list that contains null) 2. Data skew exists whereby the number of matching rows is very large (for instance, filtering for a particular foreign key value that occurs many times)

Below is my Trigger:

Trigger CaseCustomer on Case (before insert,after update) {
    Try{
    Set<String> stemail = new Set<String>();
    List<Customer__c> custlst = new List<Customer__c>();
    Map<String,Customer__c> mpemailtocustomer = new Map<String,Customer__c>();
    List<Case> updcaselst = new List<Case>();
    Map<String,Entitlement> mpentitle = new Map<String,Entitlement>();
    Set<Id> casid = new Set<Id>();
    List<CaseMilestone> cmsToUpdate = new List<CaseMilestone>();
    
    if(Trigger.isAfter)
    {
        for(Case icas:Trigger.New)
        {
            if((trigger.newMap.get(icas.id).Status!= trigger.oldMap.get(icas.id).Status) && icas.Status == 'Closed')
            {
                casid.add(icas.id);
                System.debug(casid);
            }
        }
        if(casid.size()>0)
        {
            cmsToUpdate = [select Id,completionDate from CaseMilestone where caseId IN :casid];
            System.debug(cmsToUpdate);
        }
        if(cmsToUpdate.size()>0)
        {
            for(CaseMilestone icasn:cmsToUpdate)
            {
                icasn.completionDate = System.now();
            }
            update cmsToUpdate;
        }
        
    }
    
   else
   {
    for(Case icas:Trigger.New)
    {
        if(icas.SuppliedEmail != null)
        {
            stemail.add(icas.SuppliedEmail);
            System.debug(stemail);
        }
    }
    if(stemail.size()>0)
    {
        custlst = [SELECT ID,Email__c FROM Customer__c WHERE Email__c IN:stemail];
        System.debug(custlst);
    }
    if(custlst.size()>0)
    {
        for(Customer__c icus:custlst)
        {
            mpemailtocustomer.put(icus.Email__c,icus);
            System.debug(mpemailtocustomer);
        }
    
    }
    List<Entitlement> entlst = [SELECT id,Name FROM Entitlement];
    if(entlst.size()>0)
    {
        for(Entitlement ient :entlst)
        {
            mpentitle.put(ient.Name,ient);
        }
    }
    
        for(Case icas : Trigger.New)
        {
            if(mpemailtocustomer.containskey(icas.SuppliedEmail))
            {
                icas.Customer__c = mpemailtocustomer.get(icas.SuppliedEmail).ID;           
            }
            if(icas.Origin == 'Email' ||  icas.Origin == 'Phone' || icas.Origin == 'Web')
            {
                icas.EntitlementId = mpentitle.get('Response').Id;
                System.debug(icas.EntitlementId);
            }
        }
    
    }
    }catch(exception e)
    {
        System.debug(e.getlinenumber()+e.getmessage());
    }
}