MimeCast put my mail into a perpetual greylist.
Perpetual Greylist?? Greylisting just means to refuse everything on first contact to see if the try resending, because supposedly many(/most?) of the naughty mail servers don't do resends...to speed up Directory Harvesting I suppose..
Honestly it sounds like they got a semi blacklisted IP address from their ISP. I had that happen here when we switched ISP. Our mail servers previously pristine reputation (by IP) went out the window the instant I put it behind the new guilted by association because it was in an ISP customer address block IP address. MX-Toolbox said the address was "clean" but we were still getting rejections from (see above...
) other mail systems. After much - panicked 3am class digging - I found the system that had us listed as guilty by association and forwarded that info to the ISP who swore (front line support) that the address wasn't blacklisted. This resulted in me swearing, mild hostility (3am...), the finding of a supervisor...who dug up an engineer to "prove me wrong".
Thankfully the engineer was not nearly as retarded as the rest of the staff...and therefore was able to grasp the gravity of the situation, actually read the information I'd sent, and then contacted the listing company to have the address (actually a CIDR block of 16 addresses) removed from the naughty.
And the mail began to flow...