I've been sending email through authsmtp.com servers over the last few months, to get around the Charlotte TW Telecom server which has been (irrevocably?) flagged as a spam relay by a few blacklists, something which mail servers aren't supposed to blindly follow but a lot of them do anyway. This has been good enough for most places I send mail to, but apparently not with LJ. It seems that host names starting with "outmail" don't look sufficiently like mail servers to count in S25R's rule book.
Fortunately, I have options. I switched mail to livejournal.com addresses to be sent there directly, but that didn't work as my server's reverse DNS looks even less like a "real" mail server. I then switched to the TW Telecom server, and it started working. Here's hoping that LJ's operations team doesn't get the same bad idea that other mail administrators practice to add to this one.