Considering the #Yubikey 4 as some sort of generalized two-factor utility. It does appear that #Fastmail supports it.
https://www.amazon.com/gp/product/B018Y1Q71M/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&tag=guide42-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B018Y1Q71M&linkId=f0890013d8d348d9d09b736eb0c6c8ad
