How to mass Confirm Email Address in Salesforce?

1. Create Bounced Email report in Salesforce.

2. Use Dataloader or execute the below code in Developer Console.

1. List < Lead > listLeads = [ SELECT Id, EMAILBOUNCEDDate   
2.                               FROM Lead   
3.                              WHERE EMAILBOUNCEDREASON = '550 5.7.1 Relaying denied' AND EMAILBOUNCEDDate = YESTERDAY   
4.                              LIMIT 300 ] ;  
5. for ( Lead obj : listLeads ) {  
6.       
7.     obj.EMAILBOUNCEDDate = null;  
8.     obj.EMAILBOUNCEDREASON = null;  
9.       
10. }  
11. update listLeads;  

Note:
Update the report filter and SOQL filter accordingly.