List<Account> accts = [SELECT Id,Name,(SELECT Id,Email FROM Contacts) FROM Account];//all acount related contact information
   Contact con = [select id,Name,email from Contact where email='test@gmail.com' limit 1];//single salesforce Contact

    for(Account a : accts){
       for(Contact c: a.contacts){

            System.debug('All Email'+c.email);
            here i get the all accounts contact emails.
            Now how can i compare test@gmail.com with all account related contacts emails

}

    }