This is a security setting put into place by their receiving server, i.e. their settings for their email carrier and/or 3rd party protection. This tends to occur more frequently with private domains since they have custom spam filters. Additionally, each recipient can have their own personal filter settings that can lead to messages being filtered as spam.
What can you do to prevent contacts from receiving this again?
- Have the contact add your "From address" to their safe senders list, as seen here.
- Have the contacts adjust personal filter settings