I just noticed this on my own site – it seems that WordPress is assuming that the comment author is including an email address (which can be made optional). This results in a From line like “From: So-And-So <>” which is invalid, and some mail servers will reject mail on this basis.
IIRC, B2 used to use its own admin address to send all notifications, regardless of whether the comment author provided an address. Perhaps WordPress should use this as a fallback?
