Only local part is checked

While this is probably ok in normal or single domain configurations or is what most people typically need, I'd suggest adding a configuration option that instructs the plugin on what to ask the db for.

You have:
this.is_user_valid(rcpt.user, function (isValid) {

Which only gathers the user (local part) of the receipient. In my case, supporting multiple domains, I needed it to match the entire email address and did the following:

this.is_user_valid(rcpt.user.concat('@').concat(rcpt.host), function (isValid) {

There is probably a better way to do this than concatenating but I was just doing the quickest thing possible to get it working.

I'd suggest the configuration variable be named "use_localpart_only" and when false it uses the entirety of the email address instead of just the local part.

Start by locating the recipient-validation code containing this.is_user_valid(rcpt.user, ...), then inspect how plugin configuration is read and how the PostgreSQL lookup is built. Add the requested use_localpart_only option so the lookup can use either the local part or the full email address, and verify both paths with the available tests or a local database setup.

Written by the indexing model from the issue text.