Cannot manage Plesk mail server settings through OBAS control panel. Subscriptions are created with disabled mail service and there is no way to enable it in the control panel.
This is Plesk internal issue with id #PPPM-3710, which is planned to be fixed in future product updates.
Please use a temporary fix. To apply the solution, please extract the TAR.GZ file contents to /usr/local/psa/admin/ on every Plesk node affected.