Haven’t tried it myself because I’m too chicken to lose gold:
If you loot from a mailbox, and hit gold cap, what happens to the extra gold? My primary concern is receiving gold from auctions. I couldn’t find any support articles that talk about this
I can keep moving gold around but in the event I don’t’ have enough coffee one day and forget to check my wallet, I’d love to know what happens (others may want to know as well I suppose).
When at the gold cap of 9,999,999g 99s 99c on a character or near cap you can not collect gold from the mailbox or complete a trade that would put you over. No risk of losing gold in those situations.
But if you’re at cap and loot or complete quests, that gold will be lost.