Ubuntu 11.04 postfix cannot perform rDNS queries

This is bug 764096. The bug report contains a workaround, and it seems that an update to fix the issue for good will be released sometime soon. Besides for the workaround, I recommend you visit that bug page to mark the bug as affecting you, which you can do by using the green "This bug affects..." link. It is not necessary for you to post a comment on the bug page unless you have information to add.

If you're running the 32-bit version of Ubuntu, you can find out what to put instead of x86_64-linux-gnu (in the workaround) by running:

dpkg-architecture -qDEB_HOST_MULTIARCH

I forgot to mention: You might also want to subscribe to bug 764096, as then you'll be sure to receive email notifications when the bug's status changes (for example, when a fix is ready for testing, or when a fix is released) and when someone posts a comment. I've linked the bug to this question, so you may already receive notifications about happenings in that bug through this question...but these days, those notifications often misrepresent what's actually going on in the bug, due to bug 250515 in Launchpad itself.

Thanks, but I asked this question before I found a workaround and posted it in bug 764096.

Ah--you're right. It was you who found and posted the workaround! Many thanks!