From https://lists.wikimedia.org/pipermail/wikitech-l/2007-January/028391.html: "The old web interface at http://mail.wikimedia.org will redirect to the new server."
Given that mail.wikiPedia.org, which I think was its predecessor, still redirects, probably this removal was accidental and not intentional.
I killed that domain in 2014 (operations/dns 3a7f472cb3e9bcd03f0492cfdd8c0a2156f448d3). Noone has complained since to my knowledge, and I'd recommend to not reintroduce this redirect at this point. It was confusing to begin with: before that transition main mail exchangers and the mailing list service was all in the same box; these days they are (thankfully) separate, but the side-effect is that "mail" as a label is much more ambiguous. HTH!
Thanks. The rewrite rule for it still exists in exim4. We should remove that I think?
That's what I filed T280472: Figure out if we can remove legacy domain support for mailing lists for. :)
From what I gather, we are on board with removing it, so resolving this in favor of tracking the work in T280472. Feel free to reopen.