Alternativas ao SignMyEmails para Apple Mail com licença de código aberto

• Apple Mail
  Todas as plataformas Windows Mac Linux Chrome OS Android iPhone Windows Phone iPad Android Tablet Windows S Web Firefox Windows Mobile Microsoft Office Outlook Chrome Thunderbird Mailbird Apple Mail Microsoft Office 365 Outlook Gmail
• Licença Open Source
  Qualquer licença Licença gratuita Licença Open Source Licença comercial