The maximum number of characters for each
approved/blocked list in the Administration Console is 4000. If each
address or domain is 30 to 40 characters, each sender list can include
approximately 100 to 130 addresses and domains.

The maximum number of characters for all lists for each user in the Message Center is 1000.

For each address, add an additional 2 characters to get an accurate count.

If you run out of space and attempt to add another address, you receive an error similar to this:

To free up more space, delete addresses that are no longer used.

You may consider adding a domain from a user’s list to the appropriate organization-level list to improve filtering for all users, while freeing up space for that particular user.