I had an outside client report that our emails to them were being delayed by up to 8 hours. I went looking and right off the bat, in the queue viewer I see an entry for their domain with 26 messages backed up.
All messages have the same Last Error:
450 [XXXX random ACSII characters] sender <email@example.com> SPF Temporary Error
I have googled far and wide and cannot find a resolution for this. I've checked my own SPF on MXToolbox.com and it reads as good. I checked the destinations's SPF record, it was good too.
I checked the transport log and did not have an entry for these backed up messages so I am unsure of where to look next.