Hi @paryba,
Great question, and thanks for digging into this!
The X-Mailin-Tag header is indeed a supported way to add tags when sending via SMTP. While it’s not as prominently documented as the API tags parameter, it is a legitimate feature that has been part of Brevo’s SMTP relay for some time.
For reference, the official SMTP relay documentation does list tags as an available parameter 1, confirming that tagging is supported at the SMTP level. The X-Mailin-Tag header is the mechanism to pass that value when sending via raw SMTP rather than the API.
So to directly answer your question: yes, it is safe to use in production. The fact that it’s showing up correctly in your Brevo interface (as seen in your screenshot) confirms it’s being processed as expected. If you have any issues please let me know!
Hope that helps! 