after upgrade to 4.6.26 getting PHP Fatal error when starting a new mailing
This means CRM_Mailing_BAO_Mailing class is overriden somewhere. This could be due to a civicrm extension like mailjet that overrides Mailing.php

Please check which extension is overriding it and either disable that extension or update Mailing.php in it.

Let me know if it works!

Thanks.