It sounds like you might have something in the right column that is causing the block to stretch. What is the name of the email you are working on so I can take a look?
If you would like to try to troubleshoot this I would recommend removing one block on the right at a time to see at which point the block expands again. You will want to add the block you removed back and then look for any content within the block that is causing the stretch. Large photos, links, or dividing lines (----) are common.