A common question we get from our clients is: how do I prevent my emails from going into my clients' and leads' spam/junk/bulk folders? Well, we at SMPL/AJ Wilder have been battling the spam filters for years - and mostly winning, until a few months ago when for seemingly no reason, many of our business emails started landing in our clients' spam folders. Even our replies to emails that clients initiated were going into their spam. This experience is a good illustration of the fundamental rules of spam filtering: 1) there is no foolproof way to keep all of your emails "safe" from the filters; and 2) email service providers (most significantly, the big two: Gmail/Google and Outlook.com/Microsoft) are constantly tweaking their spam filtering algorithms, so the exact same email that had no trouble reaching your inbox last year, last month, or even a few days ago might become a victim today. You have to be aware of these facts, regularly monitor your reputation, and be willing to make wording or design changes to your templates to bypass the "suspicious content" filters.
You should also understand that there are several layers to spam filtering. First is the technical stuff: make sure your domain and IP address aren't on any blacklists (use this free checking tool), and get the proper sender authentication records in place to whitelabel your domain. At SMPL we assist you with these things. But then, just as important is the actual content of each email you send. Each email service provider/ESP has their own set of rules that analyze the content of emails sent to their network, and these rules or algorithms determine which emails reach your Inbox. We did some research and found a very long list of words to avoid in the subject and body content of your emails, that should serve as a good reference for all of us: Spammy Words and Phrases
We have found that Google/Gmail has been very aggressive lately with its spam classification. Below are some helpful tips for those of you who have Gmail or Google Domains email addresses, taken from https://support.google.com/a/answer/7663057?hl=en. Of course, these tips can also be helpful to your clients, employees, vendors and any other contacts who use Gmail; feel free to pass them along!