Hi,
I noticed that you wrote you upgraded to version 6.10.4
The latest version of AcyMailing is 7.5.7, so please update to the latest version
We have done large amount of fixes related to Outlook in few latest releases and you are using ancient version of AcyMailing
BTW, as far as I ca see, your Outlook version (based on the screenshots you provided) is not 2004. It is Office 365 version