This sample code given below shows how you can write Force.com Email Services to process unsubscribe requests from the email recipients.

• The code will search in the subject line for the word “unsubscribe”
• If the word is found, it will first find all contacts who match the from Email Address and set the HasOptedOutOfEmail to true
• Then it search though the Leads and look for the same email address and update the HasOptedOutOfEmail to true as well
• The code then returns true and exits

Code :

Global class unsubscribe implements Messaging.inboundEmailHandler{

Global Messaging.InboundEmailResult handleInboundEmail(Messaging.InboundEmail email,
                            Messaging.InboundEnvelope env ) {

// Create an inboundEmailResult object for returning
//the result of the Apex Email Service
Messaging.InboundEmailResult result = new Messaging.InboundEmailResult();
 
// Contact and Lead lists to hold all the updated records
List<Contact> lc = new List <contact>();
List<Lead> ll = new List <lead>();
 
// Convert the subject line to lower case, so I can match on lower case
String mySubject = email.subject.toLowerCase();
// String I am searching for in the subject line
String s = 'unsubscribe';
 
// Check variable to see if the word "unsubscribe" was found in the subject line
Boolean unsubMe;
// Look for the unsubcribe word in the subject line,
// if it is found return true, otherwise false is returned
unsubMe = mySubject.contains(s);
 
 // If unsubscribe is found in the subject line enter the if statement
 
 if (unsubMe == true) {
     
     try {
         
     // Lookup all contacts with a matching email address
         
     for (Contact c : [Select Id, Name, Email, HasOptedOutOfEmail
                        From Contact
                        Where Email = :env.fromAddress
                        And hasOptedOutOfEmail = false
                        Limit 100]) {
                        
        // Add all the contacts into the List    
                                                        c.hasOptedOutOfEmail = true;
                            lc.add(c);                                   
    }    
        // update all the Contact records
          update lc;
             }
     catch (System.QueryException e) {
        System.debug('Contact Query Issue: ' + e);
        }     

     try {
         // Lookup all leads matching the email address
     for (Lead l : [Select Id, Name, Email, HasOptedOutOfEmail
                        From Lead
                        Where Email = :env.fromAddress
                                                And isConverted = false
                        And hasOptedOutOfEmail = false
                        Limit 100]) {
        // Add all the leads to the List        
                l.hasOptedOutOfEmail = true;
        ll.add(l);
                               
           System.debug('Lead Object: ' + l);   
    }    
        // Update all Lead records in the query
        update ll;
             }

     catch (System.QueryException e) {
        System.debug('Lead Query Issue: ' + e);
        }     

    System.debug('Found the unsubscribe word in the subject line.');
 }
 else {
     System.debug('No Unsuscribe word found in the subject line.' );
 }
// Return true and exit
// True will confirm it is complete and no bounced email
// should be send the sender of the unsubscribe request.
result.success = true;
return result;
    }    
    
    // Test method to ensure you have enough code coverage
    // Have created two methods, one that does the testing
    // with a valid "unsubcribe" in the subject line
    // and one the does not contain "unsubscribe" in the
    // subject line
    
static testMethod void testUnsubscribe() {

// Create a new email and envelope object
   Messaging.InboundEmail email = new Messaging.InboundEmail() ;
   Messaging.InboundEnvelope env     = new Messaging.InboundEnvelope();

// Create a new test Lead and insert it in the Test Method        
   Lead l = new lead(firstName='Rasmus',
               lastName='Mencke',
               Company='Salesforce',
               Email='rmencke@salesforce.com',
               HasOptedOutOfEmail=false);
   insert l;

// Create a new test Contact and insert it in the Test Method  
   Contact c = new Contact(firstName='Rasmus',
                   lastName='Mencke',
                   Email='rmencke@salesforce.com',
                   HasOptedOutOfEmail=false);
   insert c;
   
   // test with subject that matches the unsubscribe statement
   email.subject = 'test unsubscribe test';
   env.fromAddress = 'rmencke@salesforce.com';
   
   // call the class and test it with the data in the testMethod
   unsubscribe unsubscribeObj = new unsubscribe();
   unsubscribeObj.handleInboundEmail(email, env );
                        
   }
 
static testMethod void testUnsubscribe2() {

// Create a new email and envelope object
   Messaging.InboundEmail email = new Messaging.InboundEmail();
   Messaging.InboundEnvelope env = new Messaging.InboundEnvelope();

// Create a new test Lead and insert it in the Test Method        
   Lead l = new lead(firstName='Rasmus',
               lastName='Mencke',
               Company='Salesforce',
               Email='rmencke@salesforce.com',
               HasOptedOutOfEmail=false);
   insert l;

// Create a new test Contact and insert it in the Test Method    
   Contact c = new Contact(firstName='Rasmus',
                   lastName='Mencke',
                   Email='rmencke@salesforce.com',
                   HasOptedOutOfEmail=false);
   insert c;
   
   // Test with a subject that does Not contain unsubscribe
   email.subject = 'test';
   env.fromAddress = 'rmencke@salesforce.com';

   // call the class and test it with the data in the testMethod
   unsubscribe unsubscribeObj = new unsubscribe();
   unsubscribeObj.handleInboundEmail(email, env );                        
   }    
   
}

Hope this helps.