Nested email distribution groups not receiving mail to a particular DL in Exchange 2010

Question

We have a distribution list (called "MasterDL", for example) that has sub-DLs inside it, as well as normal personal user email accounts alongside those nested DLs. For some reason, only the individual accounts are receiving any mail sent to MasterDL. Any members inside the sub-DLs do not receive anything sent to MasterDL in their inbox.

The server is Exchange 2010. The client being used is Outlook 2010.

How might this be occurring?

EDIT: It should be worth noting that the way the email is sent is through a VB/.NET SharePoint 2010 solution with the following relevant (pseudo)code:

System.Net.Mail.SmtpClient s;
System.Net.Mail.MailMessage m;
s.Port = 25;
s.Host = *our exchange server name*;
s.Send(m);