Check out the Recipients collection object for your mail item, which should allow you to get the address: http://msdn.microsoft.com/en-us/library/office/ff868695.aspx
Update 8/10/2017
Looking back on this answer, I realized I did a bad thing by only linking somewhere and not providing a bit more info.
Here’s a code snippet from that MSDN link above, showing how the Recipients object can be used to get an email address (snippet is in VBA):
Sub GetSMTPAddressForRecipients(mail As Outlook.MailItem)
Dim recips As Outlook.Recipients
Dim recip As Outlook.Recipient
Dim pa As Outlook.PropertyAccessor
Const PR_SMTP_ADDRESS As String = _
"http://schemas.microsoft.com/mapi/proptag/0x39FE001E"
Set recips = mail.Recipients
For Each recip In recips
Set pa = recip.PropertyAccessor
Debug.Print recip.name &; " SMTP=" _
&; pa.GetProperty(PR_SMTP_ADDRESS)
Next
End Sub