Where and how are you setting up your signature? Where are you setting its font? However... I don't have calibri here on my tablet, nor my phone, nor on my linux computers. I don't know if Mac users will have it either. Users who do have it may choose to override the fonts requested in email messages by their own preferences. So even when I am obliged to use Windows, which does have calibri, i wouldn't see calibri. All my emails come out in Segoe UI to be consistent with menus, text labels and so forth. So, is it worth the effort?