Think I figured this out. In the mojo 2.6 sample user.config is a line:
<add key="UseLegacyCryptoHelper" value="false" />
This NEEDS to be added to any existing user.config. I wasn't able to get even a fresh install (using old user.config) working unless this was added. After that was added my email worked again. A lot of trail and error to narrow down the issue but very confident this was the whole problem.