How to mass Confirm Email Address in Salesforce?

1. Create Bounced Email report in Salesforce.


2. Use Data loader or execute the below code in Developer Console. 
 
List < Lead > listLeads = [ SELECT Id, EMAILBOUNCEDDate   
                              FROM Lead   
                             WHERE EMAILBOUNCEDREASON = '550 5.7.1 Relaying denied'
                                   AND EMAILBOUNCEDDate = YESTERDAY  
                             LIMIT 300 ] ;  
for ( Lead obj : listLeads ) {  
      
    obj.EMAILBOUNCEDDate = null;  
    obj.EMAILBOUNCEDREASON = null;  
      
}  
update listLeads;
 

Note:
Update the report filter and SOQL filter accordingly.