Almost all of inbox providers 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. Having low spam score ensures that the email has the best chance to get through the filtering tools on the recipient’s servers. However some systems might be stricter than others, even if you have a low spam score, the email may still be blocked based on the sensitivity of the filters of that subscribers inbox provider.
Some recipients are able to set up their own email
filtering. If they have done this, it is entirely possible that your email will
end up in their spam box until they add your email address to their safe sender
Instructions on how a subscriber can add you to their safe sender list can be found here.