You have created all distribution groups such as Sales@yourdomain.com, Info@yourdomain.com.and so on, they work fine internally but you cant receive mail from external users.This is becasue by default out of the box in exchange 2007 and exchange 2010, all newly created exchange distribution groups are internal only, we need to enable so that anonymous ( internet ) connections can send email to that distribution group.
To fix this do the following.
1. It would seem that by default Microsoft has decided all distribution lists are enabled for internal mail only. Wise, it would been nice to be asked this when you create the group but oh well!
So we need to enable these groups for external use. 2. First, fire up the exchange management console and go to recipient configuration and then distribution groups. 3. Then select the distribution list you would like to enable, right click it and select properties. 4. Select mailflow settings, then select ?Message Delivery Restrictions? and then click properties. 5. We now need to make sure that THERE IS NOT a tick in ?Require that all senders are authenticated? Take the tick out and mail will flow externally. Mail will now flow externally and internally correctly.