Outlook account not receiving some mails

I have an IMAP account that I forward to my Outlook mail account.
Some mails never arrive in my Outlook account.
Among them are some important mails. AMong them are mails that are from senders that are in my "safe sender list". 

I have no rules set, they don´t land in the spam folder or any other folder. They just never arrive in my Outlook account.

When I forward the mails from my IMAP to my Google account, I get all the mails. So its not a problem of my IMAP account not forwarding some mails.

In simpler words:
Account A forwards all mails to account B and account C.
All the mails arrive at B, only some at C.

I found a lot of users with the same problem, but no solution.

It just seems that Microsoft very aggressively filters out mail without sending them to the spam folder. With defeats the prpuse of the spam folder.

Any idea what to do?