In my installation I firstly recognized database errors like described in CiviCRM issue CRM-19109: Draft mailings are not saved. In a fresh local installation (Ubuntu 16.04LTS, php 5.6, mysql 5.7.13, Drupal 7.50) even more database errors came up.
Digging into the MySQL database showed error messages like:
SELECT id, component_type FROM civicrm_mailing_component WHERE is_active = 1 AND is_default = 1 GROUP BY component_type LIMIT 0, 25
#1055 - Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'database_civicrm7.civicrm_mailing_component.id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
I found that since MySQL 5.7.5 the default value for "sql_mode" is "ONLY_FULL_GROUP_BY" and this causes the problems with CiviCRM database and CiviCRM SQL queries.
In my local installation I added in
… no more problems so far.
The issue is addressed here: