Most email systems have email filtering tools installed to filter out spam. An email qualifies as spam if it contains certain terms in the content or suspicious email headers, among other reasons. A low spam score ensures that the email filtering tools on the recipient’s servers will not filter out your email. However some systems might be stricter than others, and even if you have a low score, you may still be blocked based on the sensitivity of the filters of the email recipient's service provider.

Some recipients are able to set up their own email filtering. If they've done this, it's entirely possible that your email will end up in their spam box until they add your email address to their safe sender list.