Just to confirm that the documented problems with SMTP hanging in R8.5.2 and the subsequent interim fix does indeed solve the problem. The problem is described here: link
Downloading the interim fix pack from IBM (http://www-933.ibm.com/support/fixcentral/) shows the following files (for Windows platform):
interim fix: DominoServer_852IF1_W64
interim fix: DominoServer_852IF1_W32
The Server however shows:
Which is a little confusing but if your server shows the above you know you have applied the interim fix pack successfully.