We’ve noticed a trend where some customers see sudden ISP Blocks and soft bounces from Google only for specific email messages. As we do for any reports of an issue like this, we investigated the messages to help spot a root cause. 🕵️♀️️
Gmail’s bounce messages hint at issues with the content and links #
While Gmail doesn’t share all its secrets around why they might decide to deliver or block a message, their bounce notifications can provide helpful hints to help you track down the issue. In this case, looking at the event details for a message that couldn’t get delivered shows a ISP block with a bounce code that says “Our system has detected that this message is suspicious due to the nature of the content and/or the links within.”
So we went ahead and took a closer look at the content of the messages that were getting blocked—and we found a common theme: They all included free URL shorteners in their content, the most common culprit being bit[.]ly links. After removing the URL shorteners and referencing the original destination URL in the content instead, we see mail delivery return to normal.
Why inbox providers find shortened URLs suspicious #
It’s not uncommon for senders to use URL shorteners, as they’re a quick and easy way to get specific link tracking data. But do you know who also loves free URL shorteners? Spammers. That’s because these shorteners make it easy to hide malicious websites and hosted files from their victims. If you also use a commonly abused link shortener domain in your emails, receivers may choose to block your mail too, just to be safe.
Use original URLs whenever possible #
Spam filtering is a moving target, so using external domains can always have an impact on your deliverability. Your best bet is to use the original URL to link to your website or file whenever possible. If you’re looking to track links within your messages, we recommend using Postmark’s link tracking, as these use our high reputation domain to optimize deliverability.
There are other considerations that can come into play for your email deliverability, like having valid HTML in your email, using CTAs (For example, “Reset Password”) instead of writing out the URL, and including a plain text copy. We have best practices guides for transactional and broadcast sending to give you the best deliverability possible (besides using Postmark, of course!).
More resources you might find helpful: