Add a column to heroku postgres database
I presume that you have created a migration to add mainsite_message.spam to the schema. Have you made sure that this migration is in your git repository?
If you type git status you should see untracked files. If the migration is untracked you need to git add path_to_migration and then push it to Heroku before you can run it there.
So, locally I've changed my models a few times and used South to get everything working.  I have a postgres database to power my live site, and one model keeps triggering a column mainsite_message.spam does not exist error.  But when I run heroku run python manage.py migrate mainsite from the terminal, I get Nothing to migrate.  All my migrations have been pushed.