Smart Filter is one of BlueMail's server-side services. It is the machinery that sorts incoming mail before it reaches you, and features such as Screener are built on top of it.
You will find it at Settings | Global Settings | Advanced Features | Smart Filter, as a single on/off switch.
It is not the filter at the top of your message list
This is the most common confusion, and the names really are that close together. The All, Unread and Starred buttons above your message list are view filters, they change which of your already-delivered messages are shown, and nothing more. See The filter menu for those.
Smart Filter works a step earlier than that. It sorts mail as it arrives, which is why it can keep unfamiliar senders out of your way before you ever open the app, something a view filter, which only rearranges what has already landed, cannot do for you.
What happens if you turn it off
BlueMail asks you to confirm, warning that "this will disable the Smart Filter and additional features". That warning is worth taking literally: the features that depend on this service stop working too, Screener among them. If mail you expect to be screened starts arriving in your inbox, this switch is worth checking before anything else.
Keeping it on is what makes the sorting work, so it is worth leaving enabled. If mail is not arriving as expected, Not receiving mail? is the faster place to start.
On a work device
If your employer manages your device, your administrator sets this one for the whole organization and BlueMail keeps to their choice, so the row is visible for reference, and your IT team can adjust it centrally.
The neighbouring Sync Services switch is a broader version of the same idea, it governs BlueMail's server-side services as a whole, and its warning is correspondingly blunter. Both are mapped in What is in Advanced Features?.
