Apple Mail cannot send mails from iCloud account
I have Apple Mail on the iPhone, the Mac Mini, and a Macbook. I have three accounts configured, and they're synced fine across all devices - a German mail account, a GMail account, and an iCloud account). Everything works fine, with one exception: On the Macbook, when composing a mail, I can only select the GMail and the German mail account to send from, not the iCloud account). The settings in Apple Mail and the system settings are 100% the same on all devices. Sending from icloud.com works fine on the Macbook, too. It's just Apple Mail that stubbornly thinks I shouldn't send from my iCloud account.
Pointers?
MacBook Air 13″, macOS 15.2