use the preposition "post in"

[Sitifan]

Sends an e-mail when someone replies to a topic you have posted in.

Why use the preposition "in" in the above sentence?
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Thank you very much for your reply.

[Haihao]

Like "sent in", etc. = posted in here.

[Yankee]

Hi Sitifan

In this particular case, I presume that "a topic" is a forum thread, so the posts for a particular thread can be seen as being "in the thread/topic". In other words, a forum thread can be compared to a container and the posts that make up that thread are the contents "in" that container.
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"Nearly all men can stand adversity, but if you want to test a man's character, give him power." ~ Abraham Lincoln

[MrPedantic]

I would agree with Amy. As a footnote: you could also say:

1. Sends an e-mail when someone replies to a topic you have posted to.

In this case, the thread is viewed as a noticeboard to which posts are attached.

(In an accounting context, you would also "post" items "to" an account.)

MrP