What Do ISPs Really Look At In Content: Deliverability Part VI


Anti-spam content filters used by ISPs have been in development for many years now. With so many ways for spammers to get their messages across, we look at reputation based methods (hijacking or rotating IPs, rotating from address, generating new domains to remove the reputation value). However with the global change to IPv6 on the horizon, focusing on content is becoming even more important.

It’s not enough to look for some specific phrases or key words, since abusers are constantly working the anti-spam filters to see what works and what doesn’t. Filtering based on a single or few words became simply too easy to get around.

Still, there are specific factors in content filters that you can see at work on a daily basis. The details are closely guarded by ISPs as their company secret, but there are some general rules that senders should be aware when creating mass mailing message, let’s go through them!


1. HTML vs Plain Text

In general, there is no rule saying that plain text messages will get you delivered to spam, while HTML will get you inboxed, or the other way around. However, working on the deliverability side of the Internet, I can clearly see that receivers prefer HTML messages over plain text. The explanation for this is quite simple, with HTML they get much more information to go by.

With plain text what you see is what you get, a simple message the doesn’t really have to contain any specific information. By using HTML you can go much deeper, adding images, go to action buttons, specific layouts, and more. All that information can be used by an ISP to improve the work and accuracy of their anti-spam filters, so while it seems that with plain text messages you are basically judged by your reputation- therefore content has much less value in that, HTML can be worked on a lot more.

What I mean, is the effects of both. They can either better your inbox placement, despite the neutral or poor reputation of your IP in the From address (e.g. shortly after you made changes to your sending structure). Same way, bad content can get you into the spam folder or even blocked, regardless of the perfect reputation on your sending IP.

I always recommend going for HTML instead of plain text. Of course, there are some exceptions based on the nature of the content you are sending. But lets face it, plain text works well only with official and transactional sends where everything needs to be transparent, sending marketing emails should automatically make you go for HTML, with much more complex structure and layout in order to make your message look much more attractive.


2. Text to link ratio

It is easy to see it in action. Sending a message with multiple links and very little text is an easy way to get your message delivered to spam, same as linking every line of text in your message (not to mention that just looks ugly).

This is one of the most basic things that an ISPs’ filters look at. The text to link ratio needs to be reliable, and relatable – providing actual good content value to your subscribers.  Sending them only clickable links is usually not one of them.

Additionally, this point is intertwined with the reputation of linked domain, so ISPs will allow a lot less links from a domain with poor reputation, that was already associated with abuse mailing, than they would for a domain with good reputation. What’s more there seem to be some limit even to the good ones!

The rules seem to be quite simple. Stick to one link for a short message (a few lines of text), or one per paragraph. As I was working out the filters I noticed that usually they have no issue in recognizing blog or site updates (e.g. first paragraph of an article, or just the title with a link to full article). ISPs seem to be rarely punishing those senders for these types of links (if they do, it’s usually a domain reputation issue), so if that is your business you don’t really have to worry!

If you do advertise products to your subscribers, you should take it easy on the call to action buttons. I would say that 2 well placed links will do the job just fine.


3. Image to text ratio

The same way link ratio matters, ISPs also look at the ratio between images and text. This seems to be more of a space-ratio kind of thing. The advised dimensions are usually given in the 60%-40% or 70%-30% ratio (with the text being the highest value). Nevertheless I have seen messages going for 50-50 doing quite well, so I would not stay true to just one value here. This is something that you need to test for your individual business.

One thing I can say for sure, is that having a message that is only an image is not a good idea. Yes, you can make it work, but with stellar reputation on all other values (IP, from, domain), but you can be sure your messages will be investigated closely either way. For ISPs a full image message instantly triggers a warning flag (If someone is using images only, it usually means that they want to hide their content).

Having an image with text in it is also very bad. And ISPs do usually catch on to that (don’t ask me how, but they do), so whatever you can pass on in actual text – do so. It’s better to simply write it instead of sending it in the image form and getting flagged.


4. Constant layout

Here, we are going into a subject that is closely connected to my first article on content. The templates you are using can easily identify you, and ISPs are aware of that. To explain it in plain language – your template can be understood as a kind of fingerprint that identifies you as a sender.

Of course it is not the only information they track, changing the from address or sending IP can set you back a bit, reputation wise, so does changing your template often. ISPs like consistency. This also includes your content, and of course they fully understand that you need to change and improve, but changing each and everyone one of your message can be taken as a bit of an excess.


Till next time!

Listed above are general rules that you should know while working in the email marketing industry. Adjusting your sends to fit these overall rules should improve your inbox placement and keep it in there.

Next time we will talk go into specific details of best practices in email marketing. I’ll do a breakdown of the most popular words and phrases, and focus on what effect they can have on your results, so stay tuned! 🙂

In case you missed previous posts, here they are:

Email Deliverability From A To Z Guide

Email Deliverability From A To Z

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