We’ve got a public repository and we have for it pipeline configured completely through the UI, that does not read steps from the repo at all.
The only step that it has is to trigger pipeline for our private repo, which reads steps from private repo and does stuff that requires extra accesses - like deploying app to target environments and so on.
But every time we merge contribution from someone outside of the team, we get this error in pipeline for public repo:
ERROR Unrecoverable error, skipping retries
FATAL Failed to upload and process pipeline: POST https://agent.buildkite.com/v3/jobs/id_here/pipelines: 422 Builds cannot be triggered from this pipeline as there is no Buildkite user linked to this build. Ensure that a user has added "github-email-of-contributor-here" as a verified email address on their account.
Error: The command exited with status 1
I reached out to support and Jessica and Tim said that they will disable this check for us for now.
Unfortunately, the check is organization-wide and cannot be disabled just for one pipeline, but they also said that they are looking for the better solution.
Tim asked me to also create a thread here, so other people can find it and contribute for the discussion, so here it is. 
Somewhat related: https://github.com/buildkite/feedback/issues/313