The uppercase `SMTP` address is the primary address, while lowercase `smtp` are aliases.  You can find the value in active directory in **"User > Attribute Editor > proxyAddresses"**.
``` powershell
Get-AzureADUser -ObjectId "user@domain.com" | Select -Property ProxyAddresses
```

!!! example "Example Output"
    ``` powershell
    smtp:alias@domain.com
    smtp:alias@domain.onmicrosoft.com
    SMTP:primaryaddress@domain.com
    ```