Powershell: Select a Property of a Property

I am doing a script that creates a csv of distribution group memebership. A snippet:

$list = Get-DistributionGroup -Identity grp1 | 
        Get-DistributionGroupMember |
        Select-Object -Property Name,Alias,ExternalEmailAddress,RecipientType |
        Add-Member -MemberType NoteProperty -Name Group -Value "grp1" -PassThru

$list += Get-DistributionGroup -Identity grp2 |
         Get-DistributionGroupMember |
         Select-Object -Property Name,Alias,ExternalEmailAddress,RecipientType |
         Add-Member -MemberType NoteProperty -Name "Group" -Value "grp2" -PassThru

The ExternalEmailAddress property is an object. It outputs a sting in the format of "SMTP:jdoe@contoso.com". There is a property of the object called AddressString that just has jdoe@contoso.com. That is what I would like to grab and export. So my thinking is to find the correct syntax for this command:

Select-Object -Property Name,Alias,ExternalEmailAddress.AddressString,RecipientType

Is there a way to do this directly with Select-Object? If not, what is the best way to get the result I want: A csv with the email field that isn't prepended with "SMTP:".