In many cases these days, many more cases than a lot of senders want to admit, delivery problems at the big ISPs are a result of sending mail recipients just don’t care about. The reason your mail is going to bulk? It’s not because you have minor problems in your headers. It’s not because you have some formatting issues. The reason is because your recipients just don’t care if the ISP delivers your mail or not.
A few years ago the bulk of my clients hired me to do technical audits for their mail. I fixed a lot of delivery problems that way. They’d send me their email and I’d run it through tools here and identify things they were doing that were likely to be causing problems. I’d give them some suggestions of things to change. Believe it or not, minor tweaks to headers and configuration actually did make a lot of difference in delivery.
Over time, though those tweaks less effective to fix delivery problems. Some of it is due to the MTA vendors, they’re a lot better at sending technically correct mail than they were before. There are also a lot more people giving good advice on the underlying structure and format of emails so senders can send technically clean email. I started seeing technically perfect emails from clients who were seeing major delivery problems.
There are a number of reasons that technical fixes don’t work like they used to. The short version, though, is that ISPs have dealt with much of the really blatant spam and they can focus more time and energy on the “grey mail”.
This makes my job a little harder. I can no longer just look at an email, maybe run it through some of our tools and provide a few suggestions that fix delivery problems. Delivery isn’t that simple any longer. Filters are really more focused on how the recipients react to mail. That means I need to know a lot more about a clients email program before I can even start to identify what might be causing the delivery issues.
I wish it were still so simple I could give minor technical tweaks that would appear to magically improve a client’s delivery. It was a lot simpler process then. But filters have evolved, and senders must evolve, too.

filters • headers • mime • MTA • Technical