[Dovecot] Truncated emails in Dovecot (v1.0.13)

[Iain]

Odd situation started recently whereby emails that are delivered into a 
maildir folder are being truncated when read. The emails are delivered 
via Exim and I can see the emails are complete when I look inside them. 
However, when viewing the emails via Thunderbird or a webmail client, 
they are truncated.

*** START 'dovecot -n' ***
# 1.0.13: /etc/dovecot/dovecot.conf
base_dir: /var/run/dovecot/
log_path: /var/log/mail.log
info_log_path: /var/log/mail.log
protocols: imap imaps pop3 pop3s
ssl_cert_file: /etc/ssl/server_certs/mail.example.com.crt
ssl_key_file: /etc/ssl/server_certs/mail.example.com.key
ssl_cipher_list: ALL:!LOW
disable_plaintext_auth: no
login_dir: /var/run/dovecot//login
login_executable(default): /usr/libexec/dovecot/imap-login
login_executable(imap): /usr/libexec/dovecot/imap-login
login_executable(pop3): /usr/libexec/dovecot/pop3-login
login_greeting: Welcome to the example IMAP server
first_valid_uid: 8
mail_access_groups: mail
mail_location: maildir:/home/vmail/%d/%n/mail
mail_debug: yes
mail_executable(default): /usr/libexec/dovecot/imap
mail_executable(imap): /usr/libexec/dovecot/imap
mail_executable(pop3): /usr/libexec/dovecot/pop3
mail_plugins(default): quota imap_quota
mail_plugins(imap): quota imap_quota
mail_plugins(pop3): quota
mail_plugin_dir(default): /usr/lib/dovecot/imap
mail_plugin_dir(imap): /usr/lib/dovecot/imap
mail_plugin_dir(pop3): /usr/lib/dovecot/pop3
imap_client_workarounds(default): delay-newmail outlook-idle 
netscape-eoh tb-extra-mailbox-sep
imap_client_workarounds(imap): delay-newmail outlook-idle netscape-eoh 
tb-extra-mailbox-sep
imap_client_workarounds(pop3): outlook-idle
pop3_uidl_format(default):
pop3_uidl_format(imap):
pop3_uidl_format(pop3): %08Xu%08Xv
pop3_client_workarounds(default):
pop3_client_workarounds(imap):
pop3_client_workarounds(pop3): outlook-no-nuls oe-ns-eoh
namespace:
   type: private
   separator: .
   location: maildir:/home/vmail/%d/%n/mail
   inbox: yes
auth default:
   passdb:
     driver: sql
     args: /etc/dovecot/dovecot-sql.conf
   userdb:
     driver: sql
     args: /etc/dovecot/dovecot-sql.conf
   socket:
     type: listen
     master:
       path: /var/spool/mail/dovecot/auth-master
       mode: 511
       user: root
plugin:
   trash: /etc/dovecot-trash.conf
   quota: maildir:storage=102400
*** END ***

I've deleted the dovecot cache and rebuilt, but there's no change here. 
This is running on a Gentoo Linux server (Xen) with kernel 2.6.20.

Any ideas?

Iain.