I have to send Single mail to Opportunity Owner and Update Flage to true after sending mail which default is false.

Code is something like that:FOR READIBILITY I OMITT UNNECESSARY CODE ;

trigger ExposureAssessmentQuestionnaireAlert on Opportunity (before update) {

String MailSubject='Reminder For Exposure Assessment Questionnaire';


    Set<String> UpdatedState=new Set<String>();
    Set<String> UpdatedStage=new Set<String>();
    Set<Id> Recordtypeid=new Set<Id>();
    Set<String> Opptype=new Set<String>();
    List<Decimal> UpdatedECV=new List<Decimal>();
    Set<String> PriorityIndustry=new Set<String>();
    Set<Id> OwnerIdSet=new Set<Id>();
    Set<Boolean> emalSent=new Set<Boolean>();
    List <Opportunity> opptyList = new List<Opportunity>();
       for(Opportunity oppnew:Trigger.new){
  
           UpdatedState.add(oppnew.INTL_Status__c);
           UpdatedStage.add(oppnew.StageName);   
           Recordtypeid.add(oppnew.RecordTypeId);
           Opptype.add(oppnew.OPPTY_TYPE__c);
           UpdatedECV.add(oppnew.EST_CHARGE_VOL__c);
           PriorityIndustry.add(oppnew.IND_MAP_TBL__c);
           OwnerIdSet.add(oppnew.OwnerId);
           emalSent.add(oppnew.EAQ_Email_Flag__c);
           opptyList.add(oppnew);
       }
       
       
   
    List<RecordType> rt=[select id,name from RecordType where Id in:Recordtypeid AND SobjectType='opportunity'];
   
    Set<String> RecordTypeName=new Set<String>();
    for(RecordType rtypename:rt){
   
        RecordTypeName.add((String)rtypename.name);
           
    }
   
    List<IND_MAP_TBL__c> IndustryNameId=[Select id,Name from IND_MAP_TBL__c where Id in:PriorityIndustry];
   

    List<String> IndustryName=new List<String>();
    //Set<Id> IndustryId=new Set<Id>();
   
    for(IND_MAP_TBL__c imt:IndustryNameId){
   
        IndustryName.add((String)imt.name);
        //IndustryId.add((Id)imt.id);
   
    }
     
    List<User> OwnerEmail=[Select Email from user where Id in:OwnerIdSet];
     
    String[] OwnermailId=new String[OwnerEmail.size()];
    
    integer i=0;
   
    for(User eId:OwnerEmail){
   
       OwnermailId[i]=(String)eId.email;
       i++;
    }
    
       if((UpdatedStage.contains('4. Formal Offer') || UpdatedStage.contains('5. Verbal Win')) || (!UpdatedState.contains('Rejected'))){
     
           if(RecordTypeName.contains('MA B2B Opportunity') || RecordTypeName.contains('MA B2C Opportunity') || RecordTypeName.contains('MA Prospecting Opportunity')){
          
           System.debug(industryNames.indexOf(IndustryName.get(0).trim().substring(IndustryName.get(0).trim().indexOf('-')+1,IndustryName.get(0).trim().length()))>-1);
          
           System.debug(industryCode.indexOf(IndustryName.get(0).trim().substring(0,IndustryName.get(0).trim().indexOf('-')-1)));
          
               if(Opptype.contains('Standard') && UpdatedECV.get(0)>1000000.00 && industryNames.indexOf(IndustryName.get(0).trim().substring(IndustryName.get(0).trim().indexOf('-')+1,IndustryName.get(0).trim().length()))>-1 && industryCode.indexOf(IndustryName.get(0).trim().substring(0,IndustryName.get(0).trim().indexOf('-')-1))>-1){
              
                                   
                 
                                 
                  EAQReminderToMBD.sendmail(OwnermailId,MailSubject,Open45Body);
                  
                       opptyList.EAQ_Email_Flag__c=true;
                     
                  
                  update opptyList;
                  
              
               } 
      
      
      
           }
      
   }
    else if(UpdatedStage.contains('6. Complete') && (!UpdatedState.contains('Rejected'))){
  
       if(RecordTypeName.contains('MA B2B Opportunity') || RecordTypeName.contains('MA B2C Opportunity') || RecordTypeName.contains('MA Prospecting Opportunity')){
          
           if(Opptype.contains('Standard') && UpdatedECV.get(0)>1000000.00 && industryNames.indexOf(IndustryName.get(0).trim().substring(IndustryName.get(0).trim().indexOf('-')+1,IndustryName.get(0).trim().length()))>-1 && industryCode.indexOf(IndustryName.get(0).trim().substring(0,IndustryName.get(0).trim().indexOf('-')-1))>-1){
                  
                   
                                 
                  EAQReminderToMBD.sendmail(OwnermailId,MailSubject,Signed6CompleteBody);
                  
                   opptyList.EAQ_Email_Flag__c=true;
                  
                 
                  
                  update opptyList;
              
           }//inner if
      
     

       }//middle if
  
  
  
   }//outer if
   
 
   
   
   
}

With Regards

Prabhash Mishra