The Email dilemma
Following on from my investigations into services that summarise DMARC reports, I've come to the conclusion that the reason for the warnings about email from hotmail.com is because even though the account on hotmail is configured to send as @gardiner.net.au, it still is sending the email officially using the @hotmail.com sender address.
While it passes the SPF rule (that permits the hotmail email servers to send @gardiner.net.au emails), this explains why DMARC warns that the email is "unaligned" – eg. the email says it's from @hotmail.com instead of @gardiner.net.au.
The problem is that while Hotmail does support adding additional email accounts, it only lets you configure ability to pull emails (via POP3) – you can't enter a SMTP server to send emails. Sent emails always go out via the Hotmail SMTP servers.
One solution might be to migrate the Hotmail user over to GMail and configure GMail to pull their @hotmail.com email instead (and unlike Hotmail, GMail can be configured to use a different SMTP server for addtional email accounts).
But just as I was considering this option, I came across the news that Google is discontinuing support for Exchange ActiveSync! Why does this matter? Well as a Windows Phone user this has the potential to be a show stopper. ActiveSync is the protocol used to sync my GMail, Contacts and Calendar between my phone and Google. Whilst it does say that existing configured devices will continue to function, too bad if I change phones in the future.
So another option might be to switch my domain's email over to Hotmail instead of staying with GMail, unless Microsoft can release updates for Windows Phone that restore compatibility with GMail after January 31st.