How can I train Thunderbird's spam filter?

Solution 1:

One tedious, per account, way would be to set up a filter matching that phrase and marking emails with it as junk.

Tools > Message Filters

Select the account you want to set the filter for. (This is the tedious part because this method isn't something you can currently apply globally.)

Click New...

Give your filter a name.

In the filter rules, set:

For incoming messages that:
Body: contains: Approved VIAGRA Store

Perform these actions:
Set Junk status to: Junk

Click OK to save and close out.

Now, incoming mail will be filtered for that and marked accordingly if it matches the rule.

Solution 2:

Install Junquilla and you can see an analysis of exactly why a message was marked spam or not spam.