[rabarberski]

A (very) late update.
I finally had some time to look into the details of all the great feedback you have all provided.

Originally Posted by lane

This is going to be fixed, but they are having some trouble doing so. See the writeup on the issue by the Microsoft chap who works with these things: Why does my email from Facebook, that I forward from my outlook.com account, get rejected?
Particularly, follow the comments to see the current status of the fix. I am eagerly awaiting it myself.

We are 1 month later but, according to the comments in the linked Microsoft blog post, no fix available. Unfortunatly.

Originally Posted by lane

Conclusion: you need to either wait out the fix, or do something else:

1. You could set the spam score required to move something to Spam much higher. Downside might be extra spam in your Inbox.
2. You could have Fastmail POP the mail down from Hotmail instead of using forwarding. Downside: you no longer get nearly instant delivery.

I am still in the "wait out a fix" mode. POP-ing is no solution (too much delay being added), raising the spam score will result in too much false positives.

Originally Posted by n5bb

The ugly truth is that forwarding in this manner just isn't compatible with current techniques for spam prevention. So the best solution is to log into the sources of those emails and change the email address they use to your FastMail address. I also suggest that you use a subdomain address for two reasons:
...
So if you owned the email address rab@example.com, you could use linkedin@rab.example.com as a subdomain address. The received message would by default be delivered to rab+linkedin@example.com, so if you had a "LinkedIn" folder it would receive those messages.

I've considered this before. But I like the simplicity of my current workflow where I have one (hotmail) address that I use for all web services that do not need to contact me personally. Switching to the scheme you suggest would require me to "remember" which email address i use for which service. For some it will be easy (like linkedin), for others it might get fuzzy (when the service changes name etc.)

Originally Posted by n5bb

So it's better long term to host your domain with an email provider (such as FastMail) which supports these standards ...

I agree. However, I still like the fact that, when I tell people my email address, they can easily remember or write down the email domain I am using because it is such a common one.

Originally Posted by BritTim

In theory, custom sieve code could be used to resolve the problem completely. It could check for the specific case of "known sender" being ignored for DKIM reasons, and apply a higher threshold for spam in that situation. Bit messy, of course.

Custom sieve seems like a good solution, as I am forwarding from my hotmail account to a specific fastmail alias (something like myspamhotmail@rab.fastmail.com). So I can identify from which hotmail account it was forwared from easily.
I've never used sieve scripts. And I didn't want to switch to it because it used to be that you either used the all sieve solution, or the web gui solution. I do have the feeling this has now changed, and that it is possible to both keep using the GUI for simpel rules (like filing into folders) and extra sieve rules for tweaking.

Question 1: Can anybody confirm that you can now use the GUI + sieve, before I make any breaking changes? (I didn't find anything about this in the the linked Fastmail help )

Question 2:Can anybody provide any clue on how such a sieve entry would look? And would it go under "### 3. Sieve generated for spam protection" before the actual spam score check?