Get a bop - email rejected at someone else's server - cause, SPF.
Huh? Out bound farm has been stable for more than 3 years!? (None added/removed in the IP pool.)
Dig the TXT record - yup, that's right.
Call the remote site: whistle cycle on hold for a bit, get a tech that can talk to a senior admin
about the issue. What's your allocation? I tell 'em. What's the IP of the server? Tell 'em.
I ask, what's the last received line leaving? He tells me - yep, it's one in the SPF record.
"So why is the SPF check failing?" I ask. "Surely you aren't using SPF checks past
the first received line?"

Dead silence for awhile. I ask, "Are you inspecting SPF past the first received line?"
More dead silence.
"I'll have someone get back to you." he says.

I'll just bet you will, and I'll hold my breath waiting.

About an hour later email to them stops bouncing. No call back.