There's a bug in the control file where php-pspell (available in stretch with php version 7.x) is a mandatory dependency.
The following packages have unmet dependencies: kolab-webclient : Depends: php-pspell but it is not installable E: Unable to correct problems, you have held broken packages.
That's simply not true.
Roundcubemail checks if there's the pspell library loadable, and will fall back to the Googiespell engine.
Please fix the control file and remove the defaults.inc.php file change:
/usr/share/roundcubemail/config/defaults.inc.php.dpkg-dist:// - 'pspell' - requires the PHP Pspell module and aspell installed /usr/share/roundcubemail/config/defaults.inc.php.dpkg-dist:$config['spellcheck_engine'] = 'pspell';