Too many recipients

message.too_many_recipients · HTTP 400

What it means

The injection names more than 20 recipients, the same limit mail meets on port 25, through to or through the message's To, Cc and Bcc headers. Nothing was stored.

What to do

Inject the message in batches of at most 20 recipients, using to to name each batch.

Every error the API returns is a JSON problem (RFC 9457) whose code names it and whose type links to its page here. See the docs for how the API works.