Since the update to release Undermine the mail icon has not been showing up in the warband/character select screen.
I’ve noticed that on my ‘auction house’ toons, if their inbox is full nobody else’s icon will show up…might just be a capacity problem? but by all means report it, perhaps it’s actually broken.
That’s what made me notice it because my AH toons mail was full prior to update and the icon disappeared. Logged in and everything was in the mail - put it up and the other toons’ mail icons had not appeared. Logging into the other toons I knew would have mail they also had mail waiting.
I know it goes based off of server and as you stated if one is full it will not show the others - but since the update none of them have showed for me on the server I play on mainly.