- The shared mailbox needs to be set as a remote mailbox on-prem? Is it? If not provision from on-prem
Enable-RemoteMailbox "SharedMBX" -RemoteRoutingAddress "SharedMBX@Company portal .mail.onmicrosoft.com" -alias SharedMBX - Give the shared remote mailbox permissions to send to that DL from on-PREM ( I assume you syncing from on-prem to Azure with AADConnect) https://learn.microsoft.com/en-us/exchange/recipients/distribution-groups?view=exchserver-2019#delivery-management