Hi Pradeep,

I appreciate your effort to help me out here. Ok, our average code coverage is 90% so there doesn't seem to be a problem there. The code that is now failing was last updated over 3 months ago and I haven't changed or added any additional validation rules into our live system.

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The failing method in my test class code is:

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private static testmethod void testLeadActivity3(){
      //Create an Active Account
        Account newAccount = new Account (name='XYZ Organization');
        newAccount.BillingCountry = 'Ireland';
        newAccount.Status__c = 'Active';
        newAccount.OwnerId = '00520000001dRbbAAE';
        insert newAccount;
        
        List <Account> savedAccount = [Select Status__c from Account where name = 'XYZ Organization'];
        
        //Create Active Contacts
        Contact mycontact = new Contact (lastName='XYZ Contact',Contact_Status__c = 'Active', MailingCountry='Ireland', Email='test@testmyDeDupe.com', OwnerId = '00520000001dRbbAAE');
        mycontact.accountId = savedAccount[0].Id;
        insert mycontact;
        
        Lead l = new Lead(LastName = 'AnotherRandomname1', Company = 'Another ABC, Inc.', Email='testAnother2@testmyDeDupe.com', Timestamp_Hashed__c='12');
        insert l;
        List<Lead> savedLead = [select doNotRoundRobin__c from Lead where Email = 'testAnother2@testmyDeDupe.com'];
        System.assert(savedLead[0].doNotRoundRobin__c);
    }

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TRIGGER CODE IS:

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trigger DeDupeLead on Lead (before insert, after insert) {
  List <Lead> dupeLeads = new List<Lead>();
  List <Contact> dupeContacts = new List<Contact>();
  String emailDomainLike = '';
  // no bulk processing; will only run from the UI
   for (Lead newLead : Trigger.new) {
    if (Trigger.new.size() == 1) {
      if(newLead.Email != '' && newLead.Email != null){
        System.debug('Inside first if statement..');
        dupeLeads = [Select l.Id, l.RecordTypeId, l.OwnerId, l.Email From Lead l Where l.Email = :newLead.Email]; //Email Exact Match
        if(dupeLeads.size() >= 1){
          doDeDupeActions(dupeLeads[0].RecordTypeId, dupeLeads[0].OwnerId, newLead);
        }else{
          String domain = getDomainString(newLead.Email);
          if((!isPublicEmailAddress(domain))){
            emailDomainLike = getDomainLikeString(domain);
            dupeLeads = [Select l.RecordTypeId, l.Id, l.OwnerId, l.Email From Lead l Where l.Email like :emailDomainLike]; //Email Domain match.
            if(dupeLeads.size() >= 1){
              doDeDupeActions(dupeLeads[0].RecordTypeId, dupeLeads[0].OwnerId, newLead);
            }          
          }
        }
      }
      if(dupeLeads.size() == 0){ // If there are no matches after email check.
         
         dupeLeads = [Select l.RecordTypeId, l.Id, l.OwnerId, l.Email From Lead l Where  l.company = :newLead.Company]; //Company Only Match
         if(dupeLeads.size() >=1){
            doDeDupeActions(dupeLeads[0].RecordTypeId, dupeLeads[0].OwnerId, newLead);
         }
         
         dupeLeads = [Select l.RecordTypeId, l.Id, l.OwnerId, l.Email From Lead l Where  (l.company = :newLead.Company and l.Phone = :newLead.Phone)]; //Company & Phone Match
         if(dupeLeads.size() >= 1){
           doDeDupeActions(dupeLeads[0].RecordTypeId, dupeLeads[0].OwnerId, newLead);
         }else{
           dupeLeads = [Select l.RecordTypeId, l.Id, l.OwnerId, l.Email From Lead l Where  (l.company = :newLead.Company and l.Name = :newLead.Name)]; //Company & Name Match
           if(dupeLeads.size() >= 1){
             doDeDupeActions(dupeLeads[0].RecordTypeId, dupeLeads[0].OwnerId, newLead);
           }
         }
      }
      
       if(dupeLeads.size() == 0  && newLead.Email != null){ // Still no leads after email + company + Phone + Name Match
         //Check contacts for email domain match
         String domain = getDomainString(newLead.Email);
        if(!isPublicEmailAddress(domain)){
        emailDomainLike = getDomainLikeString(domain);
        dupeContacts = [Select l.Id, l.OwnerId, l.Email From Contact l Where l.Email like :emailDomainLike]; //Email Domain match.      
           if(dupeContacts.size() >= 1){
             doDeDupeActions(null, dupeContacts[0].OwnerId, newLead);
           }
        }
       }
    }
  }
 
 
  private boolean isPublicEmailAddress(String domain){
    if(domain == 'gmail' || domain == 'yahoo' || domain == 'hotmail' || domain == 'googlemail' || domain == 'live'){
      return true;
    }return false;
  }
 
 
  private void doDeDupeActions(Id existingLeadRecordTypeId, Id dupeOwnerId, Lead newLead){
    if(Trigger.isBefore){
      if(existingLeadRecordTypeId == '01220000000YZ4HAAW'){ //if existing record is marketing lead set to remarket
        newLead.doNotRoundRobin__c = false; //do round robin the lead
        newLead.hasDupeInMarketing__c = true; //set hasDupeInMarketing flag to true
      }else{    
         newLead.OwnerId = dupeOwnerId;
         newLead.doNotRoundRobin__c = true;
      }    
    }else{
      //Marketing Lead & PreSlaes leads should not trigger an email to the owner: SF-680
      if ((Trigger.isAfter) && (newLead.doNotRoundRobin__c == true) && (newLead.RecordTypeId != '01220000000YZ4HAAW') && (newLead.Status != 'Presales')){
      //Do something else if the owner is a Queue!    
        //try{
          sendMail(newLead.OwnerId, newLead.Id);
        //}catch(Exception e){
            //do nothing..
        //}    
      }else if(newLead.hasDupeInMarketing__c == true){//New Lead has an existing dupe in Marketing leads
        //Do something else if the owner is a Queue!    
        //try{
          sendMailToMarketing(newLead.OwnerId, newLead.Id);
        //}catch(Exception e){
            //do nothing..
        //}
      }
    }
  }
 
  private String getDomainString(String emailAddress){
    String emailDomain = null;
    String[] firstSplit = emailAddress.split('@');
  String firstone =  firstSplit[1];
  String[] secondSplit = firstone.split('\\.');
  if(secondSplit.size() >= 3 && secondSplit[1] != 'co'){ //check for e.g. uk.mydomain.com but not mydomain.co.uk
    emailDomain = secondSplit[0] + '.' + secondSplit[1];
  }else{
    emailDomain = secondSplit[0];  
  }  
  return emailDomain;
  }
 
 
  private String getDomainLikeString(String domain){
  return '%@'+domain+'.%';
  }
 
  public void sendMail(Id uId, id leadId){
  if(!(String.valueOf(uId)).startsWith('00G')){ //dont try to send a mail if owner is a Queue
      Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
    List<Messaging.SendEmailResult> results = new list<Messaging.SendEmailResult>();
      mail.setTargetObjectId(uId);      
      mail.setTemplateId('00X20000001ZZycEAG');
      mail.setSaveAsActivity(false);
      mail.setWhatId(leadId);   
      System.debug('Sending Email');
      
      results = Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
      }
  }
 
    public void sendMailToMarketing(Id uId, id leadId){
      List<Messaging.SendEmailResult> results = new list<Messaging.SendEmailResult>();
      String[] toAddresses = new String[] {'marketing@newsweaver.com'};
      Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
    mail.setTargetObjectId('00520000001d8eoAAA'); //mmoynihan
      mail.setToAddresses(toAddresses);    
      mail.setTemplateId('00X20000001DXuY');
      mail.setSaveAsActivity(false);
      mail.setWhatId(leadId);   
      System.debug('Sending Email');      
      results = Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
  }
}

The deployment logs run to almost 10,000 lines long so I won't even attempt to paste any of that up here. We're approaching out of business hours here now so I plan to try and debug by disabling validation rules to see if this gets rid of the failure and then re-activate the rules 1 by 1 to see if this gets me any closer to a solution.

In the meantime if you have any other suggestions then do let me know.

Once again, thank you so much for your help - it is very much appreciated!