If the email address on your Tumblr account is from AT&T, change it before June 30, 2017. That includes email addresses from att.net, ameritech.net, bellsouth.net, flash.net, nvbell.net, pacbell.net, prodigy.net, sbcglobal.net, snet.net, swbell.net and wans.net. If you don’t make the switch to a different email address, you will lose access to your Tumblr account.
See Tumblr Help and AT&T Support for details.