Content that causes problems.
WHAT TRIGGERS FILTERS
Excessive capitals in the subject or body Multiple exclamation marks
Urgency language: act now, limited time, last chance
Words associated with scams: free money, guaranteed, risk free
A message that is one large image with almost no text Many links, particularly to unfamiliar domains Link shorteners, which obscure the destination Poor formatting and broken HTML
WHAT MATTERS MORE THAN ANY OF THESE
Authentication and reputation. Content signals are secondary.
A well-authenticated message from a good sender with an unremarkable subject reaches the inbox.
THE IMAGE PROBLEM
Many clients block images by default. A message that is entirely one image shows nothing.
Always include the message as text.
ATTACHMENTS
Avoid them in marketing email. Link to the file instead.
Attachments raise suspicion and increase size.
LINK SHORTENERS
Do not use them. They are heavily associated with spam and they hide the destination.
Use full URLs to your own domain.
TESTING
Several services score a message against common filters before you send.
