SMTPly

SMTPly is a locally installed SMTP relay service that accepts classic SMTP mail from devices and applications and sends it through a Microsoft 365 mailbox. It includes an SMTP listener with TLS, a persistent queue with retries, an IP and sender allow list, an assistant for the Azure app registration and a live mail tracker. The service runs on your own Windows server. A license costs a one-time 199 to 499 euros net per server.