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:

perl -MCPAN -e shell
clean Mail::SpamAssassin
install Mail::SpamAssassin