"Mail importing" option is present while mail service is disabled on the Plesk server.
Plesk extension bug #PMT-4423 that will be fixed in further product updates.
Currently no workaround available, simply ignore availability of extra option. As alternative solution it is possible to hide this option by adding the following line at the bottom of /usr/local/psa/admin/conf/panel.ini file:
allowMailImport = false