I'm using s/mime. Since upgrading to enigmail 2.0 outgoing messages using identities without gpg keys are not s/mime signed by default anymore. I tried to play around with the settings and managed at least to use s/mime as default for identities /with/ gpg keys. Is there a kind of some freaky combination of settings to get identities w/o gpg key to be s/mime signed by default? Thunderbird's own "sign by default" option is getting completely ignored.
edit, after installing enigmail nightly: works better now. But still, if I unset "enable openpgp support (enigmail) for this identity" I expect not to have pgp at all and to exclusively use s/mime. But s/mime signing is not selected (which should be the default according to thunderbird's settings). Instead, the enigmail menu shows "protocol pgp/mime (auto)" selected.
This review is for a previous version of the add-on (2.0).