Yes. My girlfriend has a bellsouth.net email address (it's our ISP). Recently, Tumblr announced that anyone with an email address belonging to AT&T (and yes, bellsouth.net is owned by AT&T) will no longer be able to use said email address to log into Tumblr. I don't have a clue why, it is what it is.

So now she's forced to get another email address to log into Tumblr. I got around that by creating an email address at my own personal domain that forwards to her current bellsouth.net address, and that was good enough for Tumblr.

Enjoy your free Internet.