Managing personal Outlook.com account settings, security, and privacy
Hi @Steven Lim
Since this affects only certain senders and your storage, rules, and safe-sender settings are already correct, the messages may be getting rejected before they reach your mailbox.
Please sign in directly to Outlook.com in a browser and check the Junk Email, Deleted Items, Archive, and Other folders. Also check Settings > Mail > Forwarding and Settings > Mail > Sync email to make sure forwarding, POP, or IMAP access is not moving or downloading the messages elsewhere.
If the missing messages are not visible anywhere, please try to kindly ask the affected sender to check whether they received a bounce-back or rejection response when sending to your MSN address. Adding a sender to the safe list only helps after Microsoft accepts the message, so it may not resolve a rejection occurring earlier in the delivery process
Hope this helps, please feel free to give me an update