I just logged into my email 126 and tried it. There are many tool icons in the content bar. On the far right, there are two downward double arrows (double V upside down). When clicked, two upward double arrows (double V upside down) are displayed, and all tool icons are displayed. Then, you select your personalized signature, make it in editing state, and find an icon to set the line height (a horizontal line up and down, a double arrow in the middle).