> The original text block is about 5 to 9 Mbytes (...), it freezes 
> building the MIMEText object. Actually no MemoryError isn't raised,
> just a sudden freeze of the running thread.

Can you give more details about the freeze?
 - FreeBSD version?
 - CPU, memory?
 - Full Python version?

On "freeze", the process uses 0% or 100% of the CPU time? You can use the 
strace program to trace Python activity during the freeze.

Your might try my clone of strace, strace.py, which works on FreeBSD without 
the Linux emulation (but on FreeBSD, only i386 is supported):
   http://python-ptrace.hachoir.org/trac

> Unfortunately the test script submited does not do the same behavior,
> maybe some other things are freezing the software instead of raise the
> MemoryError.

You can try the isolate the bug? Remove some code, disable functions, etc.