If it works with junk mail, shouldn't it work with good mail as well? You'd move certain messages to a particular folder, and the email client automatically learns with every move you make, building a profile of what mail going to that folder must look like.
Does that sound too good to be true? It is not. In Opera the Bayesian engine does not only find spam, you can also use it to train filters that will automatically sort incoming messages according to your (obvious, but implicit) preferences.
It does take some training, but eventually Opera gets a good deal of the messages right and the automatic sorting -- urgent, work, newsletters, family, friends, for example -- can help you deal with the daily dose of email a lot.
To set up a smart email filter in Opera:
- Select Mail | Filters... from the menu in Opera.
- Click Filters | New Filter.
- Type the desired name of the new filter.
- Highlight Match messages where subject contains under Rules.
- Click Remove rule.
- Make sure Learn from messages added to and removed from filter is checked.
- Click OK.
Initially, the filter knows nothing and sort no mail for you. But as you train it by using its folder, Opera quickly picks up the pattern and starts organizing your mail automatically and in a very smart manner.

