Just because you’ve sent an email to x thousand people doesn’t mean they’ve received it. Even if you strip out the bounces, not all the emails actually hit people’s inboxes. Spam filters, for example, can make a huge difference between how many emails actually get before people’s eyeballs.Which is where Postmaster Tools come in. It is free and requires just a small DNS change for the domain from which you send emails. If you send emails from @LibDemBoringwire.com, for example, then (usually) whoever you bought the LibDemBoringWire.com domain name from can make the change.
Once that’s sorted, the tool gives you access to graphs such as this:
I send my emails via MailChimp and this graph shows how Google’s spam filters view the reputation of the MailChimp servers. The more yellow, or even better green, there is, the higher the chances of my emails actually being delivered.
There is a small set of other indicators the tool also tracks and they all add up to giving you a picture of how well your email sending setup is working at getting the green light from spam filters.
It also means that if you have an unexpectedly poor response to an email or a batch of complaints about something not being received, you can take a look to see if there was a deliverability problem.
The tool only uses data from Google’s systems, but the picture the tool paints is one that will be similar across other systems too. What it won’t catch, however, is the occasional very specific problem, such as the way the spam filters on CIX email addresses really don’t like emails from MailChimp.