Why are replacement tags (like $FIRSTNAME$) not filled in the preview mode of my communications??

In preview mode the replacement tags are not filled because emails, newsletters and autoresponders are sent to many clients at a time. In preview mode there is only one option/page, so the server cannot know in advance how to fill the tags. But when the messages are sent, the replacement tags are filled accordingly for each client. You can check this randomly or specifically in the Communication Log by going to CONTACTS > CONTACT NAME > Other Communications - "View All" .