Wow... amazing response to this questions. We have been researching this as well for a product. I am surprised to see that so many people have issues with emai

Wow... amazing response to this questions. We have been researching this as well for a product. I am surprised to see that so many people have issues with email delivery. Though email delivery can be delayed and in theory there are no guarantees. Have you considered using SMS or a 2FA App like Duo? They should be near instantaneous. The idea of sending this over telegram or signal is a good approach though the costs of Whatsapp messaging would be prohibitive. Even SMS will not work at scale. So really boils down to 2 things (1) Speed - can you service live with delays or provide an alternate? (2) Cost - this cannot be tied to the per message cost like transactional or promotional email/SMS. A different pricing model help.