I've filed a ticket for this issue as well.
Just realized this morning that for about the past 3 months or so, any emails I sent from fastmail to either @reply.craigslist.org or @sale.craigslist.org have not been reaching the intended recipient. I'm not getting a bounce back or any other indication that the emails are not going through. On my end, the emails appear in the Sent folder and all seems well.
Figured this out b/c I started wondering why nobody was responding to either my replies to their CL ads or to my replies to their replies of my CL ads.
Tested this out this morning using my work email account and my iCloud account and basically figured out it's a fastmail issue and not a CL issue.
Is there some setting I may have accidentally activated that would cause this issue?
I've checked Spam folders, rules, etc. and can't find anything to explain why this started happening about 3 months ago.
Obviously a bit bummed b/c I've missed out on some deals and, more importantly, missed out on selling the stuff I've been trying to sell.