So on my Samsung android phone with the latest Mullvad app, I have turned on the dns content blocking for ads, malware and tracking. I have noticed that ads are still getting through in the Outlook app. Right at the top of my inbox all the time is a big fat add that looks like another email, it's there every time.
If I enable private DNS on the device and set it to use NextDNS (where I think I have just selected the default ad blocking as far as I can remember) then that Outlook ad never appears and I have a nice clean inbox.
However oddly when using NextDNS for the dns, the bbc.co.uk website constantly redirects to bbc.com as it is somehow detecting I am not in the UK, when I just use the mullvad DNS the bbc.co.uk website loads just fine as the UK version. So somehow bbc are using DNS to detect location (didn't even know this was possible but that is obviously what it is doing because my ip in both cases was the same mullvad london server) and it clearly doesn't like NextDNS.
Not sure why mullvad ad blocking doesn't block the outlook ads, it's not like outlook is some niche app that is using some unknown ad provider and its clearly possible to easily block those ads because NextDNS works fine. I'm sure there will be other ads getting through as well if Outlook is, but this is the main one I have noticed so far in my normal use. Mullvad may need to look at their ad block list?