Seeing your recently enabled URL filtering on the appliance I would recommend you to review the below configuration is as per recommendations.
Please be aware of the following Field Notice:
The server pool used by the URL Reputation feature servers has changed. As a result, when you enable the URL Filtering feature, you may observe one of the following symptoms:
· Work queue on your appliance backs up
· A large number of 'Request already expired' entries in the web_client logs
· Alerts indicating that your appliance is unable to connect to the Cisco Web Security Service
To fix this issue, you must reduce the number of URLs sent for verification at the same time.
1. Secure Shell (SSH) into the appliance.
2. Enter the command websecurityadvancedconfig.
3. Change the value for "Enter the threshold value for outstanding requests" from the default to 5. (Default is 50.)
4. Do not change any other option.
5. Commit any/all configuration changes.
As for rate limiting you can certainly create a separate sender group for the sending server of these newsletters and apply rate limiting based off the envelope sender or maximum recipients allowed per hour. This would help reduce load on the appliance from such emails.