Mails not showing up - with different address

Hi,

i have 2 accounts for gmx. gmx.com and gmx.de.

I forward all my mails from gmx.de to the gmx.com account and it works great. But Mailspring doesnt show these Mails. I tried an Alias but it didnt work.

Even another Alias i use for the gmx.com account with an gmx.com ending isnt showing up consistently after adding it to the alias.
What can i do?

Thank you!
on another topic: I also would like one inbox for all my address (including another gmail.com account but i dont know yet how to do it.)