Spam Rate
Also known as complaint rate.
They should stay on average below 0.10% from all ISPs that provide feedback loops. Google Postmaster Tools should stay consistently below 0.30%.
Bounce Rate
The percentage of sent emails that could not be successfully delivered to the recipient's inbox.
Bounce Rate should stay below 2%.
Open Rate
The percentage of recipients who open an email compared to the total number of emails sent.
You need to determine and set your average email open rate, which will be used to monitor if any deliverability issues occur from a drastic drop in the open rate. On average you should have an Open Rate > 40%, but it will vary by industry.
Click-Through Rate
The percentage of recipients who click on one or more links within an email relative to the total number of emails delivered.
Shoot for a 4%+ Click-Through Rate.
