If your post count is below 10 posts (or 20, I can't remember the exact number) there's a chance that your post will get caught in the spam filter. The post will be approved by a moderator as soon as they see it, but if you want to avoid this completely I suggest getting a few more posts.
I know it's annoying, but it helps prevent a lot of spam!
It often depends on the presence of multiple hyper-links in a post by a relatively new user, or the usage of certain keywords. Bayesian filters (which the mod-attention filter almost definitely is) can only check for certain triggers, they can't distinguish between spam content and legitimate content.
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