In Debian Sid I am using Evo, at the moment 3.8.5-2+b1.

For the SPAM classifying to work you need:

/usr/lib/evolution/3.8/modules/module-bogofilter.so
or
/usr/lib/evolution/3.8/modules/module-spamassassin.so

AND you need either bogofilter or spamassassin installed and functioning properly, in command line mode, not daemon mode.

Then in Evolution, there is no "enabling plugins" if the "module" (see above) is there, it is available.

Then from Evolution -> Preferences -> Mil Preferences -> Junk

If you have "Check custom headers for junk" enabled at all... it ignores the "Junk Filtering Software" and its settings and will not scan for SPAM (this is often a misconfigured item)

If you *DO NOT* have "Check custom headers for junk" checked... then it is a matter selecting the Software you have installed to use and its few options.

Bogofilter option: Convert Message test to Unicode

SpamAssassin option: Include remote tests (this will make SpamAssassin more reliable but slower)

That is all I can tell you. I'll be updating to 3.10.1-2 shortly, but I'm betting more than likely it is more of the same.