What is content filtering?
ISPs use many methods to determine how messages coming into their users are handled (and individual recipients may have a degree of control over mail filtering as well).
One common method is content filtering, which is based on the content of the subject line and body of messages.
Content filters "read" incoming email messages to see if the messages match any of the rules setup through the filter. Messages that match a filtering rule will be placed in the recipient's "bulk" or "junk" folder, or the messages might be filtered out completely and not assigned to the recipients email account at all.
Beyond content filtering, your deliverability ultimately depends on a lot of different factors, like how subscribers have interacted with your messages in the past, how often you send, and the reputation of your sending domain. There are always steps you can take to improve.