Email addresses being inserted into some hyperlinks

Something weird is happening. Compare the two posts below:

The first post on our production WPMU is automatically hyperlinking when there is in the line.

The second post on our development WPMU does not hyperlink anything automatically. You can see that it only hyperlinks and not

On the production site, the hyperlink is the base URL of the network, plus a user's email address:

If I remove that user from the system, then it just inserts another user's email address, so it's not specific to that user.

The problem occurs on all sites on the production WPMU.

I've deactivated all site specific plugins on a test site and it makes no difference. The network activated plugins on the production site are the same as the development site, so they don't appear to be the problem either.

Any ideas what's going on here? It's very odd. It happens on any theme, too.

Thanks for your thoughts.