@grib There are 2 ways to do replies: via Webmention or from your feed. If the replies are in your feed, you do need to add a link to the user's Micro.blog profile when @-mentioning. For Webmentions, Micro.blog will automatically add the @-username at the beginning based on in-reply-to. (Send an email if you have other questions... I can better explain it there. Thanks!)
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