I want to ask a question to a user on Ask Ubuntu, but I don't understand how to do it. There is no email, no nothing.
Can you tell me how to do it?
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You can only directly contact another user under certain circumstances.
Right now, as you are a 1 rep user, you can't, unless you asked a question or left an answer and someone else commented on it then you can "ping" them with @ followed by their user name. For example to ping me you would write @markkirby in your comment and I would receive a notification about your comment or I would write @dannorris to ping you.
Once you have the privilege to comment anywhere (requires 50 rep) there are a few other ways to make contact using the same @ method above.
These are, for most users, the only way to directly contact someone on the site, please understand, this is not a social network and "chatting" is frowned upon (outside of chat of course), so the ways to contact someone are limited to only post interactions.
Some exceptions do exist, a mod can contact anyone, any time and some users will leave e-mail or social network details in their profiles, so it is worth a look if you really want to contact someone. Please also respect the fact that some users simply do not wish to be contacted off site.
This might seem a bit unfair to a new user but the limitations exist for a reason, these posts may help you to earn the 50 rep:
I also recommend you take the tour and check out the help pages to get a better understanding of how this site works.
StackExchange sites are intended for public questions and answers. They are not really designed for private discussion.
You can ping a specific user in comments or chat. However, they are under NO obligation to respond. They are a volunteer, and answer what they please.