Thanks logontokartik and Amit K A.

logontokartik , I had SendEmailResult in place as a Apex debug log, but since it was possible to know which Community user will hit this sendEmail piece, so I cannot use Monitoring > Debug log  to capture this info ealier. So I'm implementing some code to record  SendEmailResult  on every Community user into a Custom object and push it to production and hope it will give me some useful information to fix this issue.

 Amit K A, it's Enterprise edition and this is a code:

public static void SendMultiEmails(List<ID> recipients, ID candidate, string orgWideEmailAddress, string templateName, string[] additionalEmails, string title, string body, string creatorName, string dc, string mainNav, string qrId, string commName)
	{
		List<ID> recipients = new List<ID>();
		
		List<GSCForumSubscription__c> uSubscriptions = [Select Id, User__r.Id, ForumCategory__c, CommunityCategory__c  From GSCForumSubscription__c 
															Where ForumCategory__c =: dc.DataCategoryName];
		
		for(GSCForumSubscription__c us : uSubscriptions)
		{
			recipients.add(us.User__r.Id);
		}	
		
		List<string> bccEmails = new List<string>();
   
		List<Messaging.SingleEmailMessage> allmsg = new List<Messaging.SingleEmailMessage>();
		OrgWideEmailAddress owa = [select id, DisplayName, Address from OrgWideEmailAddress where Address = :orgWideEmailAddress limit 1];
		EmailTemplate template = [Select id, body, Name from EmailTemplate where Name = :templateName limit 1];	
		
		for(ID rcp : recipients)
		{
            string recipientName = [SELECT Name FROM User WHERE Id = :rcp].Name;
            
            //rcp isn't printing..... fix later
            string subscriptionEmailBody = 'body';
		  	//Generate the SF mail objects. 
			Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();	
	
			// Who you are sending the email to	
		   	mail.setTargetObjectId(rcp);
		   	
		   	//Where are we emailing from
			mail.setOrgWideEmailAddressId(owa.id);
            
            //What is the title?
            //mail.setSubject('Geocortex Support: New ' + commName + ' - ' + dcProperName + ' Post!');
            mail.setSubject('[Subject - ' + dcProperName + '] Re: ' + title);
			
		   	// The email body to use
			//mail.setTemplateId(template.Id);
			mail.setHTMLbody(subscriptionEmailBody);
            
		  	mail.setUseSignature(false);
		
	   		mail.setBccAddresses(bccEmails);
	   		
		   	//Set Merge Fields and object to attach activity too. If null, attachs to contact.
		   	if (candidate != null)
		   	{	
		   		mail.setWhatId(candidate);	   		
		   	}
		   	
		   	//Saves it as an activity within salesforce.
		   	mail.setSaveAsActivity(false);
		   	
		   	//Add additional emails if necessary
		   	if (additionalEmails != null)
		   	{
		   		mail.setCcAddresses(additionalEmails);
		   	}
			
			//Add the email into a mail package.
			allmsg.add(mail);
		}
		
		List<Messaging.Sendemailresult> result = Messaging.sendEmail(allmsg, false);
		
		// For debugging on SendEmail failure
		if(result[0].isSuccess() == false)
		{
			List<Messaging.Sendemailerror> error = result[0].getErrors();
			system.debug('tt77 - SendEmailInfo: ' + error[0].getFields() + ' / ' + error[0].getTargetObjectId() + ' / ' + error[0].getMessage());
		}
	}