The DSBL real-time blacklist was shutdown almost a year ago. However, their nameservers continued to answer requests until yesterday.
If you are using list.dsbl.org in your RBL settings, you will want to remove this as soon as possible.
This may cause intermittent failures and RBL lookups timeout.
On Plesk, go to Servers – Mail area and adjust the RBL lists your server is using.