Why did my email pass my spam filter and not yours?

Due to the fact that there are so many advertisers and so few people
who want to be advertised to, deliverability is an incredibly complex
topic and there are several factors involved:

1.) Email Server Reputation – Web-based email clients check
the reputation of the server sending the emails. If the servers have a
bad reputation, emails sent from them will most likely be considered spam . You can check your email server’s reputation here (https://mxtoolbox.com).

A sender’s reputation is made up of a combination of IP Address +
Domain + From Email Address so it’s important that you maintain
consistency. Some ESPs filter email by domain versus only IP address.

2.) Local Configuration Settings – Every email client and/or spam blocking application has its own local settings and configuration
tools. In other words, users who receive email have the ability to
customize their client to be strict or forgiving when it comes to spam.
In our test, we have everything set to the default settings.

3.) Blocked Senders and Address Book – Several email clients
use different levels of spam blocking based on if or if not the sender
is considered “trusted” by the receiver. If the receiver places the
sender into his or her address book, future emails are less likely to be
blocked. When the receiver places the sender in their blocked list,
future emails are blocked.

4.) Email Content and Subject Lines – Spam filters look for
spammy sounding words in your subject line and the body of your email.
Most work on a point system. The more points you have against you, the
more likely you are to be placed into spam. Unfortunately, most email
clients will already start penalizing you for using HTML within your
email.

5.) Mailing List Quality – If you have several subscribers in
your mailing list that no longer have valid email addresses, you run the
risk of getting blocklisted. This is why several email service
providers are very strict about this. There are several affordable
email list cleaning services available today and most email service
providers remove individuals on your list who no longer have valid email
addresses. With that said, it’s best to stick with one provider and/or
export your list of valid addresses if you plan to change providers.

6.) Engagement – Several ESPs keep track of the way their
users interact with their email. Factors include what messages are
opened, what messages are moved between folders, and how quickly mail is
deleted.

7.) Sending Server Configuration – Rate limiting will occur
for high complaints and poor reputation. Servers should be configured to
allow for simultaneous connections with throughput set at 20 emails per
connection. Authenticating email with SPF and DKIM is recommended.

In the end, our system can only test the reputation of the sender address, subject lines and content using the default spam settings that come standard within email clients and popular spam filters.