nuc: note why its mail path differs from ks2/ks4 (source address, not config)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nuc/usb4t-dropouts.md

@@ -82,6 +82,14 @@ zed → mail (bsd-mailx) → /usr/sbin/sendmail = msmtp → mail.lutran.fr:587 (
   `zed.rc.orig`. **No credentials in this file** — they live only in
   `/etc/msmtprc`.
 
+**Why nuc differs from ks2/ks4** (which mail via s-nail straight to
+port 25, and work fine): it is not the zed config, it is the source
+address. ks4 *is* the mail server's host (internal client,
+`mynetworks`); ks2 is a datacenter IP with a PTR. nuc sends from a
+**dynamic home IP with no PTR and no SPF alignment**, so the inbound
+MX path treats it like any stranger. Hence submission + auth here,
+deliberately different from the other two hosts.
+
 Two dead ends worth remembering:
 
 - **Port 25 is the wrong port for alerts.** Authentication succeeds,