SpamAssassin was failing to deliver emails with the following message:
SMTP connection from USERNAME lost while reading message data (header)
1H4zGX-0000KH-FH == user@domain.com R=virtual_sa_user T=virtual_sa_userdelivery defer (-24): Transport filter process failed (127): unable to execute command
To fix force a reinstall of the perl module using following commands:
# perl -MCPAN -e shell
# clean Mail::SpamAssassin
# install Mail::SpamAssassin
# /etc/init.d/exim stop
# /etc/init.d/exim start