I faced the same problem as you guies told. Don't really know if that is a bug or something else.
My configs are as below:
OS= Slackware 10.0
Arch= Intell 915 based server
I also had the same problem as above.
I did the same thind. Put a blanck on SMTPMAILHOST . and things went normal.
May be i do some research to find the why this is happening. 

--Sanjay Saha
System Administrator
sanjay@freeskills.net