Thanks bunches for this. 👍
There was a change to the hook that emailers use, you can see the list of breaking changes here: https://community.nodebb.org/topic/6583/0-9-0-breaking-changes/2
It is likely the emailer needs to be updated, or if not your emailer will need to install the latest version directly from npm, and not via the plugins page.
NodeBBs on the
master branch will report their version as v0.8.2, even though that branch is going to eventially be v0.9.0.