It seems it's a bug sometimes with trading via guild stores, when in game e-mail arrives without items in attachment.
I just had such issue - email without attachment with constantly blinking icon, unable to open and read it's content.
No help by travelling between wayshrines or other map or reloading UI. Just was getting a sound like for new email arrived, but no new emails and still not readable. I wanted to raise a ticket to in game Support but got an error that it's not possible.
Ultimately I thought I will close the game and start again - and yet this helped and attachment showed up in email correctly.
But I wish ZOS could check this as it's a bug, and it's not a first time I get this problem.
PC EU (PL): 14 characters. ESO player since 06.08.2015. Farkas finest quote: "Some people don't think I'm smart. Those people get my fist. But you, I like."