My use case:

• Sending demand letters to customers - single and mass
• Custom objects: Payment_Schedule__c contains the amount to be paid/already paid amounts. Demand_Letter__c contains the body of the Demand Letter sent as well as its PDF attachment. Both are linked to Opportunity
• User will send a Demand Letter by clicking on New button in Opportunity's related list - Single
• User will send Demand Letter by selecting Opportunities and clicking Create Demand Letter button on Opportunity list view - Mass

Single sending works absolutely fine. Steps invlolved: 1. Create demand letter (name,Page1 content,Page2 content). 2. Save. 3. After saving on clicking Generate PDF button will display the contents - page1,page2 as well as pending payment in 3rd page. 4. If it looks fine, then send an email by attaching the pdf by clicking on Send Email button. DONE.

I am encountering problem with mass email. When there are more than 10 recipients(or Opportunities) selected, I receive System.LimitException: Too many Email Invocations: 11. Below is my code:

    public void sendAction() {
        try {
            showSendBtn = false;
            rtId = [select Id,name from RecordType where name='Demand Letter_After Sending' and SObjectType='Demand_Letter_Repository__c' limit 1].Id;
            if(! mass) {
                System.debug('### (DemandLetterEmailController) SINGLE. sendAction');
                sendAndAttach(dlObj);
                dlObj.RecordTypeId = rtId;
                update dlObj;
            }
            else {
                System.debug('### (DemandLetterEmailController) MASS. sendAction');
                System.debug('### (DemandLetterEmailController) lstDLR.size-inside sendAction: ' + lstDLR.size());
                //System.debug('### (DemandLetterEmailController) lstDLR-inside sendAction: ' + lstDLR);
                arrId.clear(); // arrId contains the opp ids. Hence clear all before re-using it for storing DLR's ids
                for(Demand_Letter_Repository__c dObj : lstDLR) {
                    arrId.add(dObj.Id);
                }
                lstDLR = [select Name,Opportunity__c,Page1__c,Page2__c,To__c,
                    CC__c,OwnerId,Opportunity__r.HFI__c,Opportunity__r.HFI__r.Email__c,
                    MassEmail__c,RecordTypeId from Demand_Letter_Repository__c 
                    where Id IN: arrId];
                System.debug('### (DemandLetterEmailController) after select.lstDLR.size-inside sendAction: ' + lstDLR.size());    
                for(Demand_Letter_Repository__c d : lstDLR) {
                    oId = String.valueOf(d.Opportunity__c).subString(0,15);
                    dId = String.valueOf(d.Id).subString(0,15);
                    //System.debug('### (DemandLetterEmailController) d-for loop: ' + d);
                    if(d.To__c !=null)
                        toAddresses = new String[]  { d.To__c } ;
                    else
                        toAddresses = null;
                    if(d.CC__c !=null) {
                        if(d.Opportunity__r.HFI__c !=null) {
                            System.debug('### (DemandLetterEmailController) inside d.Opportunity__r.HFI__c !=null');
                            if(d.Opportunity__r.HFI__r.Email__c != null) {
                                if(! d.CC__c.contains(d.Opportunity__r.HFI__r.Email__c)) {
                                    System.debug('### (DemandLetterEmailController)if ! d.CC__c.contains(d.Opportunity__r.HFI__r.Email__c)');
                                    d.CC__c = d.CC__c + ',' + d.Opportunity__r.HFI__r.Email__c;
                                }
                            }
                        }
                        ccAddresses = d.CC__c.split(',');
                    }
                    else
                        ccAddresses = null;
                    
                    sendAndAttach(d);
                    d.MassEmail__c = true;
                    d.RecordTypeId = rtId;
                }
                update lstDLR;
            }
            insert lstAtt;
            insert lstTask;
        }
        catch(Exception e) {
            System.debug('### (DemandLetterEmailController) Exception-sendAction: ' + e.getMessage());
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.FATAL,e.getMessage()));            
        }
    }
    private void sendAndAttach(Demand_Letter_Repository__c dlr) {
        mail = new Messaging.SingleEmailMessage();
        System.debug('### (DemandLetterEmailController) dlr: ' + dlr);
        System.debug('### (DemandLetterEmailController) dId: ' + dId);
        System.debug('### (DemandLetterEmailController) oId: ' + oId);
        pdf = new PageReference('/apex/DemandLetterPDF?id=' + dId + '&oppId=' + oId + '&type=old');
        System.debug('### (DemandLetterEmailController) pdf: ' + pdf);
        pdf.setRedirect(true);
        
        // Take the PDF content
        if(test)  // Just to skip if called from test method
            b = null;
        else
            b = pdf.getContent();
                
        //Attach to Progress-**bleep**-Demand Letter Repository record
        lstAtt.add(new Attachment(parentId =dlr.Id,name=dlr.Name + '.pdf',body=b,OwnerId=dlr.OwnerId));
        System.debug('### (DemandLetterEmailController) lstAtt: ' + lstAtt);
        
        System.debug('### (DemandLetterEmailController) toAddresses: ' + toAddresses);
        if(toAddresses== null) {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.FATAL,'Error while sending email - No TO address specified. *** A task has been created to take hard copy ***'));
            lstTask.add(new Task(Subject='Send Hard Copy',ActivityDate=System.Today(),Status='Not Started',
                                 WhatId=dlr.Id,Description='The customer does not have email specified. Send hard copy.',
                                 OwnerId = dlr.OwnerId,ReminderDateTime=System.Now(),IsReminderSet=true));
            return;
        }
        
        // Create the email attachment  
        efa = new Messaging.EmailFileAttachment();
        efa.setFileName(dlr.Name+'.pdf');
        efa.setBody(b);
        
        mail.setSubject(dlr.Name);
        //System.debug('### (DemandLetterEmailController) toAddresses: ' + toAddresses);
        mail.setToAddresses(toAddresses);
        mail.setCcAddresses(ccAddresses);
        mail.setPlainTextBody('Find attached demand letter.');
        mail.setHtmlBody('<div align="center"><h2><b>Demand Note</b></h2></div>'+
                     '<br/><br/><br/>'+
                     'Dear Sir/Madam,<br/><br/> Find attached demand letter.<br/><br/><br/>' +
                     'Regards,<br/>For Brigade Group.<br/>' +
                     'Date:' + String.valueOf(System.today()).subString(0,10));
        
        mail.setFileAttachments(new Messaging.EmailFileAttachment[] {efa});
        System.debug('### (DemandLetterEmailController) mail: ' + mail);
        r = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {mail});
        for(Messaging.sendEmailResult result : r) {
            if(result.isSuccess()) {
                ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.CONFIRM, 'Email Sent Successfully. To: ' + toAddresses + ' Cc: ' + ccAddresses));
                dlr.Sent_Date__c = System.Today();
            }
            else
                ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.FATAL, 'Error while sending email. To: ' + toAddresses + ' Cc: ' + ccAddresses));
        }
        System.debug('### (DemandLetterEmailController) mail: ' + mail);
    }
}

Any help would be appreciated.