Well this @mention is a bit more complicated than what one might think. For example: If someone creates a post, without any @mention, then edits it, and adds a @mention — then, do you detect this, when looking at the edits, and send a @mention notification now? And what if s/he edits the post again, and removes the @mention, then, do you remove the notification? Cancel the email if it hasn't been sent yet? And if the email has been sent, and the user clicks the link, and views the post — but the @mentioned user sees no @mention because it was removed, then ... this is not good UX. Should you do something about this? Explain that the @mention got removed? Or just leave the user slightly confused?

Also, do you really want to send an email for a @mention immediately? What if the user who writes the post, keeps editing it, after having saved it? Should you wait for a minute or two, until s/he seems to be done editing it? And then generate the @mention email? Or directly, even if then an early out-of-date version of the post will be included in the email.

What if the @mentioned user is in do-not-disturb mode, then you need to schedule the notification for later.

And what if the @mentioned user is already online and sees the post with the @mention in, diriectly? Then maybe you needn't generate any notification at all. But how do you determine what s/he is reading currently?

There's a lot to think about (more than what I mentioned above) that one might not think about, ... before one starts implementing everything. — And this is just for @mentions. ... Kind of goes on like this, with every small seemingly simple feature you want to add.