Can someone tell me why my post got flagged?

The problem with https://news.ycombinator.com/item?id=25541900 is that it reads like an ad. HN users hate that. They click on things looking for interesting content, so what you want to do is post the actual interesting content.

Newsletters aren't a great fit for HN in general because it's hard to link to them. When I look at https://markjgsmith.substack.com/p/mark-smiths-newsletter-26... I see a bunch of pointers to things, but HN itself is already a bunch of pointers to things, so that's too much indirection to count as interesting. More explanations about that here: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...

What you want to do is post something with a lot of meat on the bones, so people can get into it right away and have something specific to discuss. Blog posts are better for that, especially if they go deeply into a topic. Short few-paragraph blog posts can sometimes do ok on HN, if they make a clear point that strikes a nerve, but mostly they're too shallow to resonate. Blog posts that go deep are more the sweet spot.

> As far as I am aware self posts are allowed, so what makes this particular link not allowed?

Another user gave a good answer. Parts of https://news.ycombinator.com/newsguidelines.html may be relevant as well: "Please don't use HN primarily for promotion. It's ok to post your own stuff occasionally, but the primary use of the site should be for curiosity."

and

"Please submit the original source. If a post reports on something found on another site, submit the latter."

Basically, submit the things you find that are most interesting to you. Once in a while, that might be your own stuff, but mostly it won't be. In your situation, maybe your newsletter contains the things you found interesting; instead of submitting the newsletter, submit the most interesting thing it links to.

In addition to the helpful comments already posted, questions about flags and such should be directed to the mods via the Contact link in the footer, as they'll have the most insight into what occurred. The guidelines ask that we direct questions like these to them directly:

> Please don't post on HN to ask or tell us something. Send it to hn@ycombinator.com.

https://news.ycombinator.com/newsguidelines.html

Flagging is done by users that think a particular post may not be a good fit for HN (ideally). It's likely that your post was seen by some as being too promotional for yourself so they may have flagged it for that purpose. I would try re-submitting it just as a link, and to not include strings like 'My' and 'is out' in the title, instead just describe the content as succinctly as possible.