The forum software uses your email address for ID. When you make a post, the forum software sends you a notification. If you use a gmail address, the notification may contain the attached image from "registerngo". Other email services may send something similar. THIS IS NOT SPAM!
In effect, the notification of the post you made is sent to you, from you. This triggers your email service to send a "spoof" warning. Please read the message carefully, and don't just assume it is spam. If you posted a message and you get a spoof warning shortly afterwards, it is probably safe.
This does not occur with notifications of posts made by other users.