I use Outlook 2013 and I have been looking for a high quality add-in that will at least PGP sign my email. BTW, I do use HTML format setting to send email.
Failing that, what is a good GUI application that I can use for PGP?
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There are two software stacks implementing OpenPGP with Outlook 2013 Plugins:
The Gnu Privacy Guard GnuPG, which is free software. Gpg4win bundles the software for Windows, and adds some GUI clients and an Outlook plugin:
Gpg4win runs on Windows XP, Vista, 7 and 8. Both 32 and 64bit systems are supported.
The Outlook plugin GpgOL is compatible with Microsoft Outlook 2003, 2007, 2010 and 2013 (only 32bit!). GpgOL supports MS Exchange Server in Outlook 2010 and 2013 only.
The last sentence seems to be read "MS Excchange Server is only supported by GpgOL in Outlook 2010 and 2013, not the earlier versions", the German translation is a little bit more specific here.
Symantec Desktop Email Encryption subseeds the original PGP implementation (they bought it) and also claims to offer Outlook plugins. I don't know about their pricing.
Generally HTML mails should be supported by both, but I neither send HTML mails nor do I use Outlook, thus I can't give any personal feedback here.
Take a look on gpg4o. It is also GnuPG based and IMHO the best OpenPGP Implementation for Outlook