Blocked Senders list

"Blocked Senders" only sends email from individual senders to Spam and it's limited to 250.  A better solution is to use Filter Rules to block (Spam or Discard) email from multiple senders.

Webmail > Settings icon   >  Settings > Inbox > Spam settings > No Spam filtering > Filter Rules > Add new rule      See Filter email using the sender's displayed name

Note: Filter Rules do not apply to email caught by a Spam filter.   Turn off Webmail Spam filters to ensure all email received is filtered by your Filter Rules.

The Cox website referenced above also indicates:

    "View the list of up to 250 email addresses or domains you've blocked..."

However, "Blocked Senders"  doesn't provide functionality to block (send to Spam) email from an entire domain.  Only individual senders can be included.

Even when the Webmail Spam setting is set to "No Spam filtering", existing "Blocked Senders" will still send email received from those individual senders to Spam.  Filter Rules that apply to any of those Blocked Senders will also run.  If your Filter Rule Action is File into Spam, one copy will go to Spam.  But if your Filter Rule Action is File into any other folder, Blocked Senders will send one copy to Spam and your Filter Rule will send a second copy to that folder.

Identify common factors for accounts in your “Blocked Senders” to be the Condition for Filter Rules that allow multiple senders to be blocked with the same rule.  Create a Filter Rule with that Condition and an Action of  "File into” Spam.  Then delete those items from ”Blocked Senders”.  When comfortable it works, change the Filter Rule Action to "Discard".  Create similar Filter Rules for remaining items in "Blocked Senders" and delete from "Blocked Senders".  You will no longer see Spam from those senders.  Continue to create additional Filter Rules as needed.