Hello sparc.
SecuMail will not decode MIME right now. Some of our beta testers have used it with Outlook and they reported they would get an encrypted rtf or html attachment with their encrypted email, both of which SecuMail handles just fine. I don't know if this is an option in the PGP plugin or in newer versions of Outlook.
That said, adding MIME support makes sense, so we'll add this to our wish-list of features for SecuMail and hopefully we can add support for it in a future release.
As far as a plugin, if you mean on the Desktop version of Apple Mail, there's an Open Source solutions out there called GPGMail:
http://www.gpgtools.org/gpgmail.htmlNotice we have nothing to do with that product, it's just something I ran across while developing SecuMail.
Within the device, this is not possible at the moment, as Apple does not provide any facilities to write plugins for the built-in Mail app.
Right now, SecuMail integrates as tightly as it's allowed. If Apple provides a better way to integrate with the Mail app, we'll certainly update Secumail to take advantage of it.
Thank you again for purchasing our product, and feel free to contact us if you have further questions.
Ernesto.