This is difficult to explain, but you can see an example here:
You'll see that whenever @lincoln.ac.uk appears either in the text or as a mailto: link, the base URL plus a user's email address is hyperlinked to the text.
You'll see 'http://blogs.lincoln.ac.uk/amakrzanowska%40lincoln.ac.uk/' appearing as a link, but at no point have I created and inserted that link. Something is doing it automatically. If I delete that user, another user's email address appears instead, so it's not related to any user account.
Text referring to other domains, such as @lncd.org or @blah.org have no effect.
This happens with any theme on any site on the network. I've turned off all site specific plugins and it still does it. I've turned off all non-essential network activate plugins and no difference. I've replicated the plugin activation on our development server and that network is NOT affected by this issue.
I'm not sure what else to do.
Thanks for any advice.