Right after we announced this site, we were informed that the subscription function wasn't working. Every time someone attempted to sign up, they encountered a "failed to send a magic link email" error.
After some troubleshooting, we discovered that we were using Mailgun's sandbox mode, which only allows sending emails to verified addresses. We were dumb enough not to test it with ordinary emails.
We set up SPF and DKIM as quickly as possible at 10 PM to transition Mailgun into production mode, but it still refused to function. This issue cost us an additional 30 minutes before it occurred to us: "Maybe we need to upgrade the account to a paid plan?"
Then it worked. It seems it always comes down to money.