Yahoo considers any third-party applications that use password-based authentication to be "less secure" and blocks access to them by default. Since Nylas Mail still connects Yahoo accounts with stored passwords, you'll need to enable access in your Yahoo settings before connecting to Nylas Mail.
How do I enable N1 access?
Or, if you are logged into Yahoo already,
Should I be concerned with Nylas Mail's security?
No, "less secure" is not the same as "insecure". We take your security and privacy seriously and we ensure that your account credentials are encrypted and securely managed.