How to auto-populate email based on user name

Hi Mahua,
How are you distributing your form? If you are using Qualtrics email (or personalised links through another email client) to distribute it, Qualtrics will already know your recipients and when they open the form you can use Eembedded Data in the Survey Flow to capture their name and email for use in your survey project. You can display the email address (and name if you wish) back to recipients as Piped Text in the survey. This assumes that the user name and email belong to the person completing the form.
Alternatively (and regardless of whether the email and user name belong to your survey respondent), is the user name the beginning of the email (I.e. User name 'smith7' becomes email 'smith7@domain.com')? If so, you could use embedded data again to record the answer choice for your first question (Embedded Data field called User name) and then a second embedded data field where you add that with the domain to make the email (so Embedded Data field User email that is equal to User name + @domain.com).
Third option, you could use Loop & Merge - this would be particularly useful if after displaying the email, you need to ask a question associated with the email (but should also work with a descriptive text question type). You would enable Loop & Merge in the question after you capture the email, telling it to loop based on the previous question. In the list of Field 1 options you would have all the user names available in the first question, then in the Field 2 list you would spell out all the corresponding email addresses. You can then use Field 2 as your piped text value in the question to display the email address. Loop & Merge is a bit tricky with reporting so I'd suggest checking out the support page for it and trying it with some test responses first to make sure it reports in a usable way for you.
Final and most complicated option would be to use branch logic in the survey flow or display logic on the survey to write 30 rules of "if X user name selected, display X email", "if Y user name selected, display Y email", etc.
Hopefully one of these options helps for you!
Cheers,
Ashleigh