The PureMessage console becomes unresponsive when the Block list configuration file becomes too large.
Known to apply to the following Sophos product(s) and version(s)
PureMessage for Microsoft Exchange 3.1.0
All supported operating systems
What To Do
You should ensure that your Block list does not exceed 2000 entries.
Normally you would not need to maintain such a large list because Sophos maintains the realtime lookup service Sophos Extensible List (SXL).