Hi,
The mails with the sign-in code are sent from a generated @boostmails-marketing.boostnote.io address. But the domain: boostmails-marketing.boostnote.io doesn't exist and has a good chance of being rejected/purged by the receiving MTA's.
E.g. my own PostFix configuration doesn't allow incoming mails from non-existing domains (best practice, not willing to change it for this).
Excerpt from postfix log:
boostmails-marketing.boostnote.io>: Sender address rejected: Domain not found
Solution would be to create a DNS record for this, so it's resolvable.
Most users would probably use the integration with Google and such. But for the few who doesn't, they may need to go through some hoops to receive the sign-in mail from you guys.
For the rest, nice product! Happy user here.