Is there any add-on to enable asking a user to answer a question?

Question

I'm thinking an option for registered users to politely ask some other user to provide an answer

Comments

Idea: They could post on the other user's wall to inform him.

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I was thinking too about a plugin, because the plugin could also be "smart" and suggest the user which are the "experts" for that type of question (maybe looking at the answers given in the past to the questions with the same tags or within that same category)

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1. How will you prevent spam?
2. What is the exact difference between this feature and sending a private message to the user?
3. What does "politely" mean in your question?

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1. By making it cost some reputation points / or by setting a threshold (user with less than X amount of points can't ask other user for answers)
2. I was thinking we could present a list of suggested users (as max pointed out)
3. By politely I mean no more than once a day and allowing the recipient to receive a digest of the request they receive (instead of multiple emails).

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Kai, posting on the wall would requiere copy & pasting the question link, painful ux approach particularly from a tablet/mobile. I'd rather click ask user, then get a field where I start typing Q2 then get a list of all users starting with Q2 and I select Q2Apro.com :)

I just saw this on Quora (ask to Answer)