Updating non-Feeds created node with email import results in duplicate nodes

I have created nodes manually (without Feeds) and need to update the information in them using Feeds (specifically, by having users send emails with the new information, which is then parsed using Mailhandler).

When users send the emails, the messages contain the nid of the previously created node that needs to be updated. I extract the nid from the email using hook_feeds_parser_sources_alter, and then map the extracted nid to Node ID using Feeds UI. However, when running cron to update the nodes, this method results in a database error ("Integrity constraint violation"), presumably because it is for some reason trying to add a new node (rather than update it), and since the nid already exists in the 'nodes' table as a primary key it fails.

Similarly, mapping the extracted nid to a GUID representing the nid of the node also results in duplicated nodes, rather than updating the original node.

In addition to other methods described in the forums here, I have tried the methods in the following issues, without success.
http://drupal.org/node/661606
http://drupal.org/node/897258

Updating the original nodes works fine when using CSV import; however it results in duplicate nodes every time with email import.

Am I missing something?