So I solved that.
File to alter is /usr/share/kopano-webapp/client/kopano.js
Line 974 is now changed to:
Zarafa.common.ui.htmleditor.Fonts=function(){return{getFonts:function(){return{1:8,"1.1":9,2:10,"2.1":11,3:12,4:14,5:18,6:24,7:36}},getFontSizeString:function(){var a=this.getFonts(),b=Object.keys(a).sort(),c="";Ext.each(b,function(b){c+=a[b]+"pt "});return c.trim()},getFontFamilies:function(){return"Calibri=calibri,candara,segoe,optima,optima ui,arial,sans-serif;Courier New=courier new,courier;Tahoma=tahoma,arial,helvetica,sans-serif;Times New Roman=times new roman,times;Verdana=verdana,geneva"},getDefaultFontSize:function(){var a=
this.getFonts(),b=container.getSettingsModel().get("zarafa/v1/main/default_font_size");return a[b]+"pt"}}}();Ext.namespace("Zarafa.common.ui.htmleditor");
“Calibri=calibri,candara,segoe,optima,optima ui,arial,sans-serif” Is where Arial was before. Arial is one of the possible substitutes for Calibri if that is not available at the target. That does the job!