This metric counts the total number of distinct addresses which sent emails to this inbox in the time period covered by the report.
In other words, it's the count of the total number of different, individual addresses which you received an email from, either directly or by being included in the CC/BCC field.
It's a good indicator of the total number of different people who are contacting you.
You may see a percentage which reflects the change in this number from the previous time period - this can either be green if a greater number of different senders have been in touch (35% seen above), or red if it has decreased:
There will also be a highlight if one of your metrics has shown an improvement, and this can be the Senders metric:
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