This happens as the request URL that is constructed exceeds the "Maximum Query String" setting in IIS of 2048 characters.
The setting can be modified via IIS Manager as outlined below:
- On each SMEX server open Control Panel -> Administrative Tools -> IIS Manager
- Browse to 'SMEX Web Site'
- Double click on 'Request Filtering', and then the 'Edit Feature Settings...' link on the right hand side
- Increase the "Maximum query string (Bytes)" field to 20480
- Leave all other parameters at their default values
The new setting should take effect immediately, allowing you to release up to 999 messages from quarantine at a time.
Note: This fix works for FireFox, Chrome and MS Edge browsers.
As Internet Explorer truncates the URI to 2083 characters, it will still only resend about 100 messages.