Nobody is able to guarantee that messages will always reach main inbox. Adding appropriate headers influences message’s placement in receiver’s inbox, however again does not guarantee reaching the main inbox. By adding “list” header we mark a message as transactional, which should be placed in the main inbox. In case of marketing send out a “bulk” header should be used instead of “list” – this will protect our marketing content form being placed in a spam folder. Instead, in the worst case scenario, it should reach „offers” or “other” bookmarks.
„Precendence” headers are currently only supported by Gmail.