"We do track open rates for emails that are already HTML formatted and making remote requests for images, but you’ll never get 100% accuracy with that metric because many people use plain text emails or don’t load images. Our experience is that the best you’ll ever see is between 60-70% “open” rate because of this. Some of our applications in some contexts only send plain text emails as well, so we don’t track open rate there at all.

Why is remote server acceptance rate important anyway? Some thoughts:

1) First, because hard bounces really do happen a lot, and at our scale, a 1% difference in hard bounce rate means 160k messages per week that aren’t making it to users, which means a poor experience for many and many support requests coming in to us. Based on all the information we’ve been able to find, we’re pretty sure a 0.7% hard bounce rate at our scale is pretty good.

2) Second, because it is a relative metric of overall deliverability. Our experience has been that when we do get on a blacklist, servers start hard bouncing our mail until we get off of it. As we’ve improved our SpamAssassin type scores over the last few years, we’ve seen an improvement in hard bounce rate. We also see a strong correlation between hard bounce rate and the number of email delivery related support requests we get. While it’s not the perfect measure, it’s the best measure we have available that we can reliably monitor.

Again, there’s no perfect way that I know of to reliably tell whether an email is getting to a user, since read status isn’t particularly accurate. We use whatever we can (hard bounce rate, open rate, number of support requests relating to email) to get as close to that as we can."