This is a bug in the way mime email is encoded. Jeff fixed it, and the fix will go into OpenACS 5.1.2.
If you would like to fix the problem before then, you can copy acs-tcl/tcl/html-email-procs.tcl or something like that from the oacs-5-1 branch off of CVS.
As I already installed OpenACS5.1.1 on my machine, I copied only the /acs-tcl/tcl/html-email-procs.tcl from 5.1.2 and replaced with 5.1.1 existing code. But still I am facing the same problem.
Could you please let me know what else I have to copy to make this work?