AT&T RBL blacklisting

I've been running a mail/web server for about 12 years on Linode, have SPF and DKIM set up, and everything works fine, except that for the last year or so email to AT&T servers bounces back with

xxx@sbcglobal.net: host ff-ip4-mx-vip2.prodigy.net[144.160.159.22] said:
553 5.3.0 flpd584 DNSBL:RBL 521< [my server IP here] _is_blocked. For assistance forward this error to abuse_rbl@abuse-att.net (in reply to MAIL FROM command)

Anybody else have this problem? Wondering if it's a Linode IP-block blacklist, although I suppose if it was I would have seen it in a search for posts. When I check public RBLs like at https://www.dnsbl.info/ everything comes up green/unblocked for my IP.

When I email that abuse-att.net address I get an autoresponder saying "When the AT&T Postmaster staff receives a request with the needed information requests are evaluated. If granted, the block will be removed within 24 to 48 hours." I've emailed them in June and October and got no further reply other than that autoresponder, and the blacklisting is still in place. Tried again just now but don't have high hopes, so just checking here if any others have experience with this, or suggestions on how to resolve/remove.