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.
What Can I Do About Content Filtering?
There are things that you can do to reduce the likelihood of your messages being content filtered, as well as things you can do to resolve any filtering that may occur.
The first thing you should be doing is using our spam-checking tool to sniff out any content you have that may be likely to trigger a filter. Learn more about it here.
For more advice, check out the following two posts on our blog:
Content Filtering Basics.
Addressing False Positives.