I checked the Content page for comments the last few days and saw the huge amount of spam comments accumulated for moderation. Mollom had been catching spam comments and holding them for moderation and after deleting the existing ones to see how much spam was arriving I realized I was getting at least 500 spams every day. The recommended setting for each content type is to discard comments which are detected as spam, and I may have configured Mollom to hold them for moderation to check its efficacy, or it may have been the previous default.
On viewing the unapproved comments I realized that Mollom is quite accurate in its spam detection. But rather than configure Mollom to delete them I have configured Mollom to use reCaptcha. This seems to have stopped the spams outright, but I am waiting to see if the spammers can defeat the reCaptchas. Using the reCaptcha option by default appears to disable the settings for deciding what to do if spam is detected, as though Mollom assumes that entry which passes the reCaptcha test can't be spam. Perhaps those that beat the captchas will be few enough to be moderated manually, and they may get a higher rank if they are reported as spam.
The downside is that when reCaptcha is enabled caching is turned off and it might undermine my sites ablity to handle a Slashdotting. I am tempted to post an interesting article on HN or Reddit to see how much my 512MB Digital Ocean VPS will handle it. Perhaps I ought to trick out Drupal with Varnish, memcache and all that stuff.
Normally when you delete spam comments after reviewing them on a one-by-one basis you have the option to notify Mollom on your reason for deletion so the content can be used to improve the spam detection algorithms. But what do you do when you need to delete them in bulk, as this removes the ability to notify Mollom and thus improve their detection rates? As it turns out the Mollom module as from version 7-2.10 now has the option to notified Mollom when spam is deleted in bulk, so the content from the hundreds or thousands that you can delete at a go will help improve it now.