Hi @suiopo , thanks for posting on this community!
This is related to the new requirements related to Yahoo and Gmail senders.
In short:
- from February 1st, Yahoo and Gmail have implemented new sender requirements
- as you’re sending emails from an @gmail.com address, you have to meet them
- you can’t send emails if your domain is not authenticated
We did a full post on this topic, visible here: New Google and Yahoo requirements - #3 par ahudavert
Does this help?