UltimateMailMod mistakenly treats "Gold Hammer of the Boundless Dune" and "Rare Gold Hammer of the Boundless Dune" as bound and cannot send them.
There is a quick-and-dirty fix for this. Edit UltimateMailMod\Locales\EN.lua and change the value of UMM_TOOLTIP_BOUND from "Bound" to "Bound%s".