curl --request POST \
  --url https://api.lemlist.com/api/inbox/email \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "sendUserId": "usr_h47tiJr87Zn7XtQHi",
  "sendUserEmail": "[email protected]",
  "sendUserMailboxId": "usm_CE8QtNtgMjTHjfaL8",
  "contactId": "ctc_yHXD3NWjPg7Z9MuWZ",
  "leadId": "lea_7HAh3odqPKamkBTv9",
  "subject": "Follow up",
  "message": "<p>Hello,</p><p>Following up on our conversation...</p>"
}
'

{
  "ok": true
}