Fastmail support are slow but excellent. I asked about an obscure IMAP oddity that I was seeing in Mutt. They escalated to an expert who said it was doing thing

Fastmail support are slow but excellent. I asked about an obscure IMAP oddity that I was seeing in Mutt. They escalated to an expert who said it was doing things the old way and cited relevant RFCs about how to do things the new way. I managed to patch Mutt default behaviour in handling IMAP the correct way - all thanks to FM support.