[BritTim]

If Fastmail's claim that there is nothing in their logs related to the undelivered messages is accurate, this is quite strange, but surely suggests a problem at the sending end. A bad network route is one possibility. It is very hard to diagnose such issues without access to the sending server.

EDIT: Is it possible to change the email address you are using to a Fastmail domain address (presumably using the same MX servers) and see if mail is still undelivered? Fastmail support might be more interested in escalating their response if you can show your problem potentially affects everyone at Fastmail.