[exim] Delivery of emails from *.Outlook.Com

Dear list members

I am facing a strange situation and would like to get your opinion on the
matter.

My mail server is running exim v4.88 on freebsd. Sometimes my users are
receiving email where another colleague my be in to: or cc:, but the other
colleague is not receiving anything. So far it happened twice and both the
times the mails were delivered from certain outlook.com server.

For testing I sent email from Gmail and Yahoo, to one colleague and cc/to
another colleague -- they were delivered to all addressed as expected.

Based on that, my thoughts are, those outlook.com servers are actually
delivering one mail, and expecting my server to do the rest (i.e., delivery
to cc or bcc).

But of course I may be wrong, and suggestions on what else I can test will
be greatly appreciated. Also, if my server is to deal with Cc et al, what
changes do I need to make in my configuration?

Also, if anyone encountered such situation, could you please confirm if it
happened the same way? How did you deal with it?

Thanks and regards