Since version 6.1.9 Acymailing is showing borders (only top, left and bottom) around blocks. When creating the campaign the use of borders had been disabled. Yet Outlook is showing the borders. These borders are not visisble in the Acymailing preview and they also do not appear when I check the mail with K9 Mail for Android.
Workaround: leave borders unchecked in the block settings, and change the color of the border to the backgound color.