Older versions of Mailsmith incorrectly think that SpamSieve 2.8.5 and later are too old to be compatible. (This is due to a change in SpamSieve’s version number format.) You can fix this by entering this command in Terminal:
defaults write /Applications/SpamSieve.app/Contents/Info CFBundleVersion 2.8.6
assuming that SpamSieve is in your Applications folder.