Other features or issues related to Microsoft Partner Center
The issue is that emails forwarded via iCloud are encountering problems. This only affects verification codes sent from storedeveloper.microsoft.com; there are no issues with verification codes from other Microsoft sites, nor with other types of emails originating from storedeveloper.microsoft.com. It appears this problem has persisted for quite some time. It seems Microsoft has no intention of addressing or resolving it, so please be aware of this. I managed to work around it by using a different email service for forwarding.