Although no one replied to the post I thought I would post my finding in case anyone ever ran across this issue. The reason the SMB (Win-based NAS) wasn't working was because Courier fetch in Maildir format contained a colon which is not allowed in win based file names.
However on the same device there was a NFS option we attemped to use this but obviously ran into the same issue. However using Character Translation Maps in the the winblows NFS server we can trasnlate the colon to be converted to something else while on the win filesystem and converted back when read by the NFS client:
Configuring Server for NFS to Use a Character Translation File
1. Log on to the Server for NFS computer by using an administrative level account.
2. Open Services for UNIX Administration.
3. Click the Server for NFS pane, and then click the Server Settings tab.
4. Under File name handling, click to select the Translate File names check box.
5. CREATE a pain text file that simply contains a hex translation (one per line) as followings
0x00 0x3a : 0x00 0x2d ; replace client ":" with server dash "-"
Save / exit
6 . Type the full path to the translation file that you created in the Use character translation files box, or click Browse to locate the file.
7. Click Apply.
8. Change Locking time to 15seconds (Locking tab), click apply
9. Restart NFS
Although clearly not my first choice to use anything Win based for this solution, sometimes you have to work with what you have and when no one wants to shell out any money this is what it comes down too.
BTW, you can also do the following prior to building the package with courier to avoid the colon
However I chose not to go this route as this could violate standard maildir format
Hope this helps someone else.