Comment 3 for bug 1633855

Also affects Ubuntu 14.04.5 LTS.
The whole of Kontact/Kmail is broken.
The reason seems to be the mysql security updates of 2016-10-26:

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Start-Date: 2016-10-26 00:40:44
Upgrade: mysql-common:amd64 (5.5.52-0ubuntu0.14.04.1, 5.5.53-0ubuntu0.14.04.1)
End-Date: 2016-10-26 00:40:51

Start-Date: 2016-10-26 00:41:16
Upgrade: mysql-server-core-5.5:amd64 (5.5.52-0ubuntu0.14.04.1, 5.5.53-0ubuntu0.14.04.1)
End-Date: 2016-10-26 00:41:25

Start-Date: 2016-10-26 00:41:45
Upgrade: mysql-client-core-5.5:amd64 (5.5.52-0ubuntu0.14.04.1, 5.5.53-0ubuntu0.14.04.1), mysql-common:amd64 (5.5.52-0ubuntu0.14.04.1, 5.5.53-0ubuntu0.14.04.1), libmysqlclient18:amd64 (5.5.52-0ubuntu0.14.04.1, 5.5.53-0ubuntu0.14.04.1)
End-Date: 2016-10-26 00:41:53
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The log file "~/.local/share/akonadi/akonadiserver.error" says:
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Database process exited unexpectedly during initial connection!
executable: "/usr/sbin/mysqld-akonadi"
arguments: ("--defaults-file=/home/username/.local/share/akonadi/mysql.conf", "--datadir=/home/username/.local/share/akonadi/db_data/", "--socket=/tmp/akonadi-username.EZqgDd/mysql.socket")
stdout: ""
stderr: "/usr/sbin/mysqld: Error on realpath() on '/var/lib/mysql-files' (Error 2)
161027 5:22:54 [ERROR] Failed to access directory for --secure-file-priv. Please make sure that directory exists and is accessible by MySQL Server. Supplied value : /var/lib/mysql-files
161027 5:22:54 [ERROR] Aborting
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