Using the New Outlook for Mac features in business environments
Dear Nilgün Kanibir
Thank you for posting to Microsoft Community
Based on your description, I understand that you seem to be receiving emails from postmaster. If my understanding is wrong, please point it out.
May I ask if this email was received by you after you sent an email to your outlook mailbox? Can you tell me exactly what happened when you received this email?
I suggest you can click this link first(https://outlook.live.com/), try to log in your outlook mailbox in the web version, click the gear-shaped [Settings] button in the upper right corner, check whether there are any settings that do not belong to you in [Rules] and [Forwarding and IMAP] under [Mail], if so, please delete them.
Currently there are a lot of users who have reported that their mailboxes have been hacked and strange rules have been added causing them to receive emails from postmaster. As I am not quite sure what kind of action the email was sent after. So it may not have been your outlook mailbox that was set up with the strange rules. But there must have been a mailbox in the process where such strange rules were set. I would suggest you to completely check the outgoing and incoming mailboxes of your normal emails and check their rules and forwarding.
Also, since this issue is caused by hackers who have compromised the email account, I suggest you to strengthen the security of your email account.
This reply may not directly address your issue, so feel free to let me know how it goes any time by repling below.
I look forward to hearing from you!
Carmid.L-MSFT | Microsoft Community Support Specialist