I have the same issue but think the reason is something else
I read this thread and I am experiencing the same issue as described. I can reply to a hotmail address and it works, however once I try to create from the webmail and send it, it doesn't go to my hotmail inbox. I do not think this is spam, Falco. I found out something interessting.
I wrote an email to my email box on my server and also sent the email to some other hotmail account that I have. Then I did a REPLY ALL in webmail, and then it went back to the email that I sent it from (which was hotmail). Then for the other address, it now says reply to "firstname.lastname@example.org"@hostname.domainname. com, which is wrong. So, something is happening with UebiMiau Webmail, but I am not sure how to fix it.
If I use Outlook, however, everything works.
I would appreciate any comments or help on this matter,