You just installed your new HTML email signature and sent your first email. But the recipient hasn't received it. So looking into the issue, you discover it's been caught in a spam filter! It must be the email signature, right?
When your company sends enough emails, you'll inevitably run into spam filter issues at some point. Generally, your company email signature will white-list your company emails as spammers don't use an email signature for apparent reasons.
Evaluating an email delivery issue includes your email server, the recipient spam filter settings, your email subject, your email content/links/images, and your email signature. Most spam filter issues are caused by email content and are not caused by your email signature. This means that the symptom typically is related to one or a few senders/recipients.
This article will advise general Best Practice guidelines to help you troubleshoot and fix the most common spam filter issues.
How do spam filters work?
Spam filters consider a long list of criteria when judging the "spamminess" of an email. They'll weigh each factor and add them to assign a spam score, determining whether an email will pass through the filter. If the score exceeds a certain threshold, your email will get flagged as spam and go straight to the junk folder.
Each spam filter functions a bit differently. For example, your email could pass through Spam Filter A without issue but get flagged by another Spam Filter B. To simplify things, spam filters are typically configured by individual server administrators. Thus, your email could pass the spam filter in one company but get flagged as spam in another company using the same spam filter (because the two companies use different configurations).
As for that list of spammy criteria, it's constantly growing and adapting, based on—at least in part—what people identify as spam with the 'Mark as spam' or 'This is junk' button in their inbox. Spam filters even sync up to share what they've learned. There's no magic formula—spam filters don't publish details regarding their filtering practices—but there are general steps you can take to avoid landing in your recipients' junk folder.
Keep a healthy balance of text and images.
Image-heavy emails without much text can raise a red flag in spam filters. Although different spam filters have other criteria for a healthy balance of text and graphics, it is recommended to maintain a proper text-to-image ratio to ensure your emails are delivered. Aim at an 80:20 text-to-image ratio as a thumb rule to be safe (a 60:40 text-to-image balance can be sufficient).
Only use the link to web pages.
A URL path leading to a PHP page will get your email filtered as spam with 100% certainty.
Keep the number of links below 6.
When you check the emails in your spam folder, you'll notice that they often include many links. Therefore, try and reduce the number of links in your email signature. Having five links usually is not a problem.
Keep a healthy balance of the skin and non-skin colors.
Most spam filters estimate the intensity of skin and non-skin color pixels in your emails. Mostly when you use head-shot photos, it's critical to test OK for a few pilot users. We've also seen emails with campaign banners getting filtered as spam because the banner skin color ratio was too high.
Create your image path (URL).
Generally, we recommend using MySignature images to secure that they will not get your email spam filtered.
If you deal with a client that specifically white-listed your domain, then you could try and use the image path to your server:
Avoid sending default mobile signatures.
Although the "Sent from iPhone" line as your signature most probably won't be treated as SPAM by email filters, the recipient can view it as such. While some people ignore those disclaimers as a relic of the previous decade, others might view them as highly unprofessional and careless. If you care about your brand, the best idea is to unify your email signatures across various devices and email clients.
Test your email signature.
If you are unsure what is wrong with your email signature, the excellent idea is to test it thoroughly – add one element at a time, send test emails to various recipients and from different IPs, and so on. Sometimes, emails land in SPAM just because someone accidentally added you to their blocklist. It is widespread to do this on a mobile app. Surprisingly, doing so can also affect desktop email clients, even if the blacklist is not accessible from these desktop apps.