[email protected]:~> apt list --upgradable
Listing... Done
libmariadbclient18/unknown 10.1.28+maria-1~xenial amd64 [upgradable from: 10.1.26+maria-1~xenial]
libmysqlclient18/unknown 10.1.28+maria-1~xenial amd64 [upgradable from: 10.1.26+maria-1~xenial]
mariadb-client-10.1/unknown 10.1.28+maria-1~xenial amd64 [upgradable from: 10.1.26+maria-1~xenial]
mariadb-client-core-10.1/unknown 10.1.28+maria-1~xenial amd64 [upgradable from: 10.1.26+maria-1~xenial]
mariadb-common/unknown,unknown,unknown 10.1.28+maria-1~xenial all [upgradable from: 10.1.26+maria-1~xenial]
mariadb-server/unknown,unknown,unknown 10.1.28+maria-1~xenial all [upgradable from: 10.1.26+maria-1~xenial]
mariadb-server-10.1/unknown 10.1.28+maria-1~xenial amd64 [upgradable from: 10.1.26+maria-1~xenial]
mariadb-server-core-10.1/unknown 10.1.28+maria-1~xenial amd64 [upgradable from: 10.1.26+maria-1~xenial]
mysql-common/unknown,unknown,unknown 10.1.28+maria-1~xenial all [upgradable from: 10.1.26+maria-1~xenial]

[email protected]:~> apt-cache policy libmariadbclient18
libmariadbclient18:
  Installed: 10.1.26+maria-1~xenial
  Candidate: 10.1.28+maria-1~xenial
  Version table:
     10.1.28+maria-1~xenial 500
        500 http://mirrors.accretive-networks.net/mariadb/repo/10.1/ubuntu xenial/main amd64 Packages
 *** 10.1.26+maria-1~xenial 100
        100 /var/lib/dpkg/status

[email protected]:~> grep -Ei 'codename|origin' /var/lib/apt/lists/*mirrors.accretive-networks.net*
/var/lib/apt/lists/mirrors.accretive-networks.net_mariadb_repo_10.1_ubuntu_dists_xenial_InRelease:Origin: MariaDB
/var/lib/apt/lists/mirrors.accretive-networks.net_mariadb_repo_10.1_ubuntu_dists_xenial_InRelease:Codename: xenial

[email protected]:~> sudo unattended-upgrade -v
Initial blacklisted packages:
Initial whitelisted packages:
Starting unattended upgrades script
Allowed origins are: ['o=Ubuntu,a=xenial', 'o=Ubuntu,a=xenial-security', 'o=UbuntuESM,a=xenial', 'o=Ubuntu,a=xenial-updates', 'o=Google\\, Inc.,a=stable', 'o=MariaDB,a=xenial']
No packages found that can be upgraded unattended and no pending auto-removals