I am also puzzled as to why this is happening. From the email settings test, my SMTP configuration was successful. However, when used to send emails with this new feature, it always fails. I tried switching the protocol to HTTP and using the endpoint manager https://email.manager.io you provided in this post. I tested the email sending and it was successful. However, since the sender’s email is not from a domain I own, some clients restrict their email reception with a domain whitelist, and my domain is already registered. It seems rather complicated if I have to ask them to register a new domain, especially since it’s not my company’s domain. However, this indicates there is an issue with my email SMTP server. I have tried using all ports, but it still doesn’t work. For now, I might try to create my own HTTP Server. Hopefully, it will work. In the meantime, I will send the emails manually. Thanks for your effort @Lubos.