Mailbox providers use hundreds of data points regarding your email program to make decisions about whether to place your email in the inbox or spam folder. The techniques used in their spam filters to determine inbox or spam folder placement may consist of:
- An internal set of rules and thresholds based on sender reputation metrics such as complaints, spam traps and unknown users.
- 3rd party data from an IP or domain blacklist.
- A 3rd party spam filter that uses its own proprietary techniques to identify spam.
- A content-based filter that looks for spam characteristics within your email such as invalid HTML tags, invalid or broken links, and viruses.
- Subscriber actions towards your email. One subscriber may hit the spam button on your email which sends your future email to the spam folder. Another subscriber may add your address to their contact list, resulting in your email being sent to their inbox.
No two mailbox providers use the same mix of filtering techniques and 3rd party data. Subscribers may also behave differently across the mailbox providers based on numerous factors related to your email program. The differences in filtering methods as well as the degree a mailbox provider weights and prioritizes all of the hundreds of data points and subscriber actions may result in different inbox placement rates across mailbox providers.