Why do you call it spamming? If you send to random e-mail addresses, yes, it is spamming. But if you send to your friends it is up to them if they want to use Lightning or not. Nobody is forcing anybody. But without real world adoption, CBDC will win.
Regarding sybil attack, one way would be to only allow certain domains to send to. For example: only gmail users. Creating multiple gmail accounts is certainly possible, but considering that we will only send once to any email, an attacker will need to spend a lot of time to create several gmail accounts and the reward would be insignificant.
What worked much better (but doesnt scale):
Each christmas I give my parents btc in same fiat amount. Its been 5-6 years now so I could come to them and say: If I gave you euros, you would have maybe 25% less now because of inflation. With Bitcoin you have X
And thats undeniable. They understood....sadly they still dont have any savings to save apart from what I hold for them.
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It's up to the receiver to decide what is spam. If it's unprompted message, then it's more likely to be considered spam by the receiver.
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If those friends didnt ask for it, it is unwanted.
I come from position of someone who zaps random people left and right and I think more than half still doesnt care. They lose backup for their 5k sats on WoS so its a donation to WoS :) My sister got onchain btc in 2017....lost seed. Obiously.
I have 5 gmail accounts :)
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