apt-get output:

apt-get install sope4.9-gdl1-mysql
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
sope4.9-gdl1-mysql : Depends: libmysqlclient18 but it is not installable

sogo.log:
2016-04-23 20:11:59.200 sogod[16650] EOAdaptor: cannot find adaptor bundle: 'MySQL'
Apr 23 20:11:59 sogod [16650]: [ERROR] <0x0x55c5562cc890[WOWatchDog]> an exception occured in runloop <NSException: 0x55c5561c97e0> NAME:NSInvalidArgumentException REASON:Tried to add nil value for key 'localhost:5432:sogo_user:sogo' to dictionary INFO:{}

/etc/sogo/sogo.conf:
/ Database configuration (mysql:// or postgresql://) /
SOGoProfileURL = "mysql://sogo_user:60efbf779537e2e8fcc145b0100c45aa@localhost:5432/sogo/sogo_user_profile";
OCSFolderInfoURL = "mysql://sogo_user:60efbf779537e2e8fcc145b0100c45aa@localhost:5432/sogo/sogo_folder_info";
OCSSessionsFolderURL = "mysql://sogo_user:60efbf779537e2e8fcc145b0100c45aa@localhost:5432/sogo/sogo_sessions_folder";