In a function like :

$enveloppe['from']=$from;
$enveloppe['to']=$to;
$enveloppe['subject']=$subject;
$enveloppe['date']=date('r');
$part[0]['type']=TYPETEXT;
$part[0]['subtype']='plain';
$part[0]['encoding']=ENCQUOTEDPRINTABLE;
$part[0]['contents.data']=$some_text;
$part[1]['type']=TYPEMESSAGE;
$part[1]['subtype']='RFC822';
$part[1]['disposition.type']='inline';
$part[1]['contents.data']=$some_rfc_822_text;
// The result of imap_mail_compose() is an
// exit signal Segmentation fault: (11)
// in apache error_log
$mail=imap_mail_compose($enveloppe,$part);

Thank you for this bug report. To properly diagnose the problem, we
need a backtrace to see what is happening behind the scenes. To
find out how to generate a backtrace, please read
http://bugs.php.net/bugs-generating-backtrace.php for *NIX and
http://bugs.php.net/bugs-generating-backtrace-win32.php for Win32

Once you have generated a backtrace, please submit it to this bug
report and change the status back to "Open". Thank you for helping
us make PHP better.

And please provide the real reproduce script.
The one you've posted doesn't contain any data.

Here is the backtrace :

#0  mail_free_stringlist (string=0x14) at mail.c:5792
#1  0x00cd603a in mail_free_body_data (body=0x934b1b0) at mail.c:5651
#2  0x00cd6063 in mail_free_body_part (part=0x9075628) at mail.c:5699
#3  0x00cd6070 in mail_free_body_part (part=0x90755bc) at mail.c:5701
#4  0x00cd5fec in mail_free_body_data (body=0x9075580) at mail.c:5647
#5  0x00cd609f in mail_free_body (body=0xbf8f3c54) at mail.c:5633
#6  0x00bc03c9 in zif_imap_mail_compose (ht=2, return_value=0x925b984, this_ptr=0x0, return_value_used=1) at /web/apache/Install/php4-STABLE-200512130936/ext/imap/php_imap.c:3188
#7  0x00cc27bb in execute (op_array=0x9267ee8) at /web/apache/Install/php4-STABLE-200512130936/Zend/zend_execute.c:1675
#8  0x00cc2a25 in execute (op_array=0x91dcf40) at /web/apache/Install/php4-STABLE-200512130936/Zend/zend_execute.c:1719
#9  0x00cc2a25 in execute (op_array=0x90dd4dc) at /web/apache/Install/php4-STABLE-200512130936/Zend/zend_execute.c:1719
#10 0x00cae617 in zend_execute_scripts (type=8, retval=0x0, file_count=3) at /web/apache/Install/php4-STABLE-200512130936/Zend/zend.c:934
#11 0x00c7b6e9 in php_execute_script (primary_file=0xbf90aff4) at /web/apache/Install/php4-STABLE-200512130936/main/main.c:1753
#12 0x00cc8724 in php_handler (r=0x90d33c0) at /web/apache/Install/php4-STABLE-200512130936/sapi/apache2handler/sapi_apache2.c:581
#13 0x08087d8e in ap_run_handler (r=0x90d33c0) at config.c:152
#14 0x08088106 in ap_invoke_handler (r=0x90d33c0) at config.c:364
#15 0x0807ee6d in ap_process_request (r=0x90d33c0) at http_request.c:249
#16 0x0807a72d in ap_process_http_connection (c=0x90cf158) at http_core.c:251
#17 0x080918de in ap_run_process_connection (c=0x90cf158) at connection.c:43
#18 0x0808652e in child_main (child_num_arg=Variable "child_num_arg" is not available.
) at prefork.c:610
#19 0x080866e1 in make_child (s=0x8ff5d68, slot=0) at prefork.c:650
#20 0x080867aa in startup_children (number_to_start=5) at prefork.c:722
#21 0x08086f6c in ap_mpm_run (_pconf=0x8fbb0a8, plog=0x8ff1180, s=0x8ff5d68) at prefork.c:941
#22 0x0808c2e3 in main (argc=2, argv=0xbf90b424) at main.c:618

Please try using this CVS snapshot:

  http://snaps.php.net/php5.1-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php5.1-win32-latest.zip

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

bug in c-client mail_gc_body() function.

 case TYPEMESSAGE:		/* encapsulated message */
    if (body->subtype && !strcmp (body->subtype,"RFC822")) {
      mail_free_stringlist (&body->nested.msg->lines); <--- here
      mail_gc_msg (body->nested.msg,GC_TEXTS);
    }

nested.msg is union.

in source, comparison is uppercase, so try lowercase to subtype.

$part[1]['subtype']='rfc822';