Currently, HES supports up to 100 entries in the End User Quarantine Blocked and Approved Senders lists.
This limit has been increased to 100 from 50 entries in the previous version of HES.
If this limit is too restrictive for your users' purposes, as a workaround, an option is using an asterisk in the format when filtering all addresses from a domain or subdomain to be added to the senders' list.
For the detailed steps, refer to the HES End User Quarantine User Guide's Adding Senders section in the HES Online Help page.