Problem with accents (character traslation problem?)

I have squirrelmail installed from sources.
I have a problem in traslating characters in squirrelmail when we send a 
mail using thunderbird 2.x (with ISO8859P1 western characters), and then 
we forward or reply the mail using squirrelmail (UTF8 configuration).
The destination mail (squirrelmail or thunderbird) get bads characters 
instead of accents.
An example of this is:

 > Nestor, mi compa*&ntilde*;ero Eduardo ya instal*ó* el
 > sistema en la nueva pc, lo dej*ó* instalado en ambas pcs
 > con distinto usuario.
 > Saludos,
 > Untitled Document
 > Andr*é*s Fernando Yacopino

The problem is not present when we configured the thunderbird to use 
UTF8 instead of 8859P1.
We don't have problems between thunderbird clients.

The configuration is:

config.php:

$squirrelmail_default_language = 'es_ES';
$default_charset       = 'utf-8';
$lossy_encoding        = true;

locale/es_ES/setup.php:

$languages['es_ES']['NAME']    = 'Spanish';
$languages['es_ES']['ALTNAME'] = 'Español';
$languages['es_ES']['CHARSET'] = 'utf-8';
$languages['es_ES']['LOCALE']  = array('es_ES.utf-8','es_ES.utf-8','es_ES');
$languages['es']['ALIAS'] = 'es_ES';

/functions/i18n.php:
$languages['es_ES']['NAME']    = 'Spanish';
$languages['es_ES']['CHARSET'] = 'utf-8';
$languages['es_ES']['LOCALE']  = array('es_ES.UTF-8','es_ES.UTF-8','es_ES');
$languages['es']['ALIAS']      = 'es_ES';


The version of the software is:

-SquirrelMail Version: 1.4.16
-Php 5.2.6
-Apache 2.2.10 (Unix)
-IMap Dovecot 1.2.6
-Postfix 2.5.9
-Plugins:
Installed Plugins
    1. html_mail 2.3-1.4
    2. addgraphics
    3. image_buttons
    4. message_details
    5. newmail
    6. delete_move_next
    7. squirrelspell
    8. autocomplete
    9. address_add
    10. abook_import_export
    11. archive_mail
    12. folder_sizes
    13. mark_read
    14. check_quota
    15. show_thumb
    16. ldapquery
    17. msg_flags
    18. compatibility 2.0.13
    19. empty_folders
    20. quicksave
    21. timeout_user
    22. translate
    23. calendar
    24. smallcal
    25. view_as_html 3.8
    26. username
    27. lockout
    28. change_ldappass
    29. javascript_libs
    30. avelsieve

Can you help me?

Thanks a lot,

-- 

Andrés Fernando Yacopino

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