When you create/refresh sandbox, Email Deliverability default to **System Email Only**. **System Email Only** allows us to send emails to users or reset passwords.You can run below class, when you create/refresh sandbox.
[![enter image description here][1]][1]

 When you run below class then it will update all System Admin's user's email address and that will eventually send email to user. 

 

 global class SandboxPostRefresh_AC implements SandboxPostCopy { 
 
 global void runApexClass(SandboxContext context) { 
 
 System.debug(context.organizationId()); 
 
 System.debug(context.sandboxId()); 
 
 System.debug(context.sandboxName()); 
 
 run(); 
 
 } 
 
 global static void run() { 

 //List of all emails from the User object 

 List<User> userEmailList = [select Email from User where profile.name = ‘System Administrator’]; 

 for(User uc : userEmailList) 

 { 

 uc.Email = uc.Email.replace('=','@'); 

 //to remove appended domain 

 String addedPhrase = '@example.com'; 

 uc.Email = uc.Email.remove(addedPhrase); 

 userEmailList.add(uc); 

 } 

 if(userEmailList.size() > 0) 

 { 

 Update userEmailList; 

 } 

 } 
 } 


 [1]: https://i.sstatic.net/tmEDI.png