curl --request POST \
  --url https://api.anchorbrowser.io/v1/mailboxes/{mailboxId}/send-probe \
  --header 'Content-Type: application/json' \
  --header 'anchor-api-key: <api-key>' \
  --data '
{
  "to": "jsmith@example.com"
}
'

{
  "message_id": "<string>",
  "sent_at": "<string>",
  "probe_subject": "<string>",
  "to": "<string>"
}