Cannot send mails to my second AWS workmail mailbox

Hello,

I successfully configured my DNS settings to use it for AWS workmail. My main email address that I configured first is working perfectly! But I also added a second user with his own email address. This user can perfectly login to the AWS workmail web-app and can send emails from there, but when I try to send email to the address, I immediately get the following error before my mail program can even send the mail: "Recipient address rejected: User unknown in virtual mailbox table."