Unknown mailing status on few mailings recipients

Question

Since the 4.7.8 update to 4.7.10 and 4.7.10 to 4.7.12 recently some mails are not reaching their recipients. If I look at the Mailing Detail Report the Successful Deliveries rate is between 90% and 100% and the status of the not received mails are "Unknown".

To see these mails on the report I have to filter as status "-any-" because there is no possibility to do it with the status "unknown".

Has anyone encountered the same problem? What may be due?

Edit

I investigated a little more and I see that "Unknown" status appears when there is no record on the civicrm_mailing_event_bounce table AND when there is no record on the civicrm_mailing_event_delivered. Then, it seems that for some users, there isn't registering the delivery of the mail.

I begin to suspect that the problem is related with an issue of inconsitency caches of the mailing recipients groups.

Edit 2

I found on logs messages like [info] CiviMail will not send an empty mail body, Skipping: xxxxxxxxxx@xxxxx.xxx corresponding to the contacts with mailing status marked as "Unknown".

Answer 1

This has been fixed for 4.7.15

PR https://github.com/civicrm/civicrm-core/pull/9463

Answer 2

I found the answer on this other question: Mail not send if preferred_mail_format is NULL.

The original problem seems reported and solved first on CRM-18037. But also persist and dragged in later versions as reported on CRM-18157, CRM-19135, CRM-19442, CRM-19423.

Otherwise in the latest fixes upgrade scripts that converts 'NULL' preferred_mail_formats to 'Both' are not executed. If I'm on right, last time is executed is in 4.7.7 upgrade.

Meanwhile to solve current inconsistencies it seems suficient to execute the following script on the database:

UPDATE `civicrm_contact` SET `preferred_mail_format` = 'Both' WHERE `preferred_mail_format` IS NULL;