The problem is that we rely on the PEAR DB or MDB2 libraries for database access. You can use either (controlled by the db_pear_type setting in the hastymail2.conf file) and they are easy to install via your distro package manager or using the PEAR installer. You can install the DB package to fix the problem without changing your settings, or change the db_pear_type to MDB2 and install the MDB2 package to correct database support.

Hope that helps,
Jason