To see how an email is going to be received, you can run a spam test on your emails and receive a Spam filter score. These scores show you the likelihood a spam filter will catch your email and put it in a user's spam inbox. Barracuda, MessageLabs, Spam Assassin, Outlook, and Webmail/ISP clients have their own way of handling spam filters.
Barracuda Spam Firewall
Barracuda Spam Firewall is a hardware spam filter. Barracuda reports a single score between 0 and 10 and a flag indicating if the email is considered to be spam. For Barracuda, an ideal score is below 3.5.
For some very spam-like emails, Barracuda will quarantine the email. In these cases Litmus will report the email as considered spam and mark it with a score of 10.
MessageLabs is a server-side spam filter that scores in two different ways. On most emails, MessageLabs reports the score from a configured SpamAssassin installation, but there is no threshold in this case. So even with a score a 4, the email might be marked as spam.
Sometimes, MessageLabs processes the email using its own filter, which reports a very similar score to SpamAssassin, but rounded to the nearest whole number. In both cases, a score of 3 or more is to be avoided, but Litmus will always report if the email was considered spam by MessageLabs as a separate calculation to the score.
SpamAssassin comes with a large set of rules which are applied to determine whether an email is spam or not. The scores can be positive or negative, with positive values indicating spam and negative values indicating not spam. The higher the positive score is for your email, the higher the probability that the message is spam.
Generally your email should have a score of 5.0 or lower to be considered passing. The lower your score, the less likely your email is to be marked spam.
Spam Assassin scores in Email Insights and Acoustic Campaign
Your SpamAssassin Score in Acoustic Campaign might be different from the SpamAssassin Score in the Email Insights Spam Test. The Email Insights (Litmus) version of SpamAssassin uses the latest version of the program and its default settings. Acoustic Campaign was modified to ignore some faulty error messages from SpamAssassin and may also be using a different ruleset.
Remember both scores are strictly informational and do not guarantee the deliverability of your email. It's always best to follow Acoustic Campaign Best Practices when designing your email.
Outlook utilizes a self-learning filter to determine what each user thinks is spam making it inconsistent for use across thousands of tests on our servers. Instead, we've added in hundreds of spam rules that have been published by Outlook. Whenever the content in your campaign triggers one of these rules, we'll provide you with feedback on what can be changed to make your email look less like spam in Outlook.
This filter on Litmus uses built-in junk email filter for Outlook. This has various sensitivity settings, here we have set it to 'High'. The Microsoft Outlook filter scores from 0-10 on the High sensitivity rating, with 0 being the highest (passing) and 10 being the lowest (failing). Outlook rates an email with a 6.0 or higher (out of 10) as a failure. A lower score (lower than 6.0) is considered a passing score with the High sensitivity rating.
Emails successfully received show as a "passed" score. Anything with a "failed" score indicates that the email was not received in the inbox, meaning the email could have been blocked or sent to the junk folder for that email client. Make sure you're sending the email to the complete list of seed list addresses in order to see the accurate reputation scoring for your email.
The challenge of testing for spam on webmail providers (such as Gmail, Yahoo! Mail, Outlook.com, etc) is that a large portion of their spam scoring is based on internal metrics and algorithms. For example, it's common for a single email sent to a single inbox on Yahoo! to be delivered successfully, but that same email might become suspicious when sent to ten or hundreds of thousands of recipients. Sometimes, a portion of recipients complain or don't engage with the email, leading the rest to be delayed or marked as spam. Trends in engagement from a particular sender or IP can produce blocks or delays as spam filters "learn" about what individual users consider spam through open, click, and reporting behaviors. These scenarios are not always predictable.
Webmail filters do not provide us any further information as to why an email has not been received in the inbox. Check warnings produced by server side filters (like Barracuda or SpamAssassin) and update your email with the changes those filters suggest. Also check with your email service provider to see if users for a particular webmail service have marked your previous campaigns as spam in the past, as this can affect your spam performance for your email campaigns sent now.