Add the failed/successful count to the table

https://wordpress.org/support/topic/bugs-131/#post-16744730
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Possible improvements:
Above the email list, there are three options, All / Successful / Failed. All is the total number of emails, and there is a number next to it, but there is no number next to Successful and Failed, better to add it, as it can show the email delivery status more clearly.
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