This error generally occurs when you try to send same email to two or more people at the same time. But sometimes it happens while sending to single recipient as well.

Solutions:

  1. Most of the time if you just clear browser cache and Cookies , it goes back to normal.
  2. If it doesn’t work then check for browser update, sometimes update error causes numerous problems.
  3. Check for installed extensions, disable them and restart the browser to test.
  4. Temporally disable any virus checker if you have.
  5. Start gmail without any labs you might have enabled. Try the default lab.
  6. If it is still doesn’t work then just reinstall the browser or switch to another browser.

If you want to send same email to large number of people  . Please create a group ..

Follow the bulk email sending guidelines 

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