Hey,
It's hard to say something by just knowing that after 15-20 users your application is giving the error but still I am trying to solve your problem.
Look, we can only send 500 mails using the SMTP Configuration for Gmail/ Yahoo/ Hotmail but if we are sending bulkmail from Outlook or something else like outlook then we can only send 100 mails using the SMTP Configurations.
So, it might be possible that you are sending Emails more than allowed.
But if this's not a problem than you can try my these articles, these will definitely send your Emails to atmost 500 Users:
http://www.c-sharpcorner.com/UploadFile/cd7c2e/send-bulk-email-from-yahoo-and-hotmail-using-Asp-Net/[
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http://www.c-sharpcorner.com/UploadFile/cd7c2e/send-bulk-mails-using-smtp-configuration-part-2/[
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