This metric counts the total number of distinct addresses which you sent emails to.
In other words, it's the count of the total number of different, individual addresses which you either sent an email directly to or included in the CC/BCC field.
It's a good indicator of the total number of different people you are contacting.
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 recipients have been contacted, or red if it is less.
There will also be a highlight if one of your metrics has shown an improvement, and this can be the Recipients metric.
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Very handy to see how many different people you have been sending emails to, and to compare with the number of sent emails.
For example, if you are sending emails to a large number of different people, or to a select few, this will be reflected in these two metrics.