Just like how the item restore categories were removed, the mail return feature was removed as well. The link you can find only redirects to the support landing page for all games so no help there either.
i.e https://eu.battle.net/support/en/article/6403
I have alts stuck in exiles reach with bags I would like to send back but to no avail since it takes 1h to go through everything.
There’s no information, it appears atleast removed since last September.
And one reply above the below link another poster got a reply when ticketed and basically got told that they will not accept abuse of the mailbox as 2nd storage space. Kinda like the undelete toons being used as extra space and so forth problems.