The spam filter is designed to detect spam posts and automatically remove them. However, legitimate posts are often caught by the filter. If a few minutes go by and your post isn't showing up on the new page of the community where you posted, it has probably been caught by the filter. This is most likely to occur if you are posting to a community that you have not participated in before. Each community has an independent filter, for example /r/help's filter doesn't talk to /r/pic's filter. In order to remove your post from the filter you need to message the moderators (this link can be found in the sidebar on the right-hand in that community, you can also manually compose a message to #communityname) and ask them to check the filter for you. Eventually the filter will "learn" that your posts don't need to be removed. If your posts are frequently removed in one community in particular you can send a request to the moderators to be added as an approved submitter.
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