The 'Spam Detector' will detect spam emails and then move them to the spam folder or mark them as spam.

Tap on the button in the bottom right of the screen with four boxes. Then tap on the 'Spam Filter' button to show the Spam rules. You can then turn these rules on or off and also edit their settings.

When you first use AltaMail the spam detector rule will only mark emails as spam with a flag. 🚫 You can also tap on this flag to edit the spam settings.

You can change the spam detector rule to also move suspected spam emails to the spam folder. Open the 'Spam Filter' screen and then tap on the 'Spam Detector' rule to edit it. Then turn on the 'Move to folder Spam' button.

Tap on the filter section of the spam rule to choose how spam emails are detected.

Spam Score
The spam score value changes how sensitive the spam rule is. You can change the spam score value or turn it off if good emails are being marked as spam. (More)
The spam rule may not detect all spam emails so you can also use the spam swipe action to mark those emails as spam and block them. (More)

Reject invalid DKIM signatures
The DKIM setting will reject emails with an invalid DKIM record. The DKIM record proves the email has not been altered. DKIM records may be added to the email headers by the sender's email server depending on the server settings.
This is an effective filter.

Reject invalid SPF results
The SPF setting will filter out emails that do not match the email server SPF record. If SPF is enabled for the sender's email server then the SPF record will be stored on the email server. The sending network IP address of the email is then checked to see if it is allowed by the server SPF record.
The SPF filter may reject some valid emails as some email servers have incorrect SPF records and do not include all of the IP addresses used to send emails. An example of this is when a company uses a marketing email company to send newsletters and does not include the marketing mail servers in the SPF list.
If you use this setting be sure to check your spam folder regularly to see if valid emails are moved there. Swipe on valid emails and use the 'Not Spam' action to add those emails to the safe list.

Reject invalid DMARC check
The DMARC setting will reject emails that fail the DMARC test. DMARC is a record stored on many email servers and describes what to do with invalid emails. DKIM does not validate an email but is used to check both the SPF and DKIM status of emails. Some DKIM records specify the action as 'none' which serves no purpose as it allows all emails.
This is an effective filter but does rely on the accuracy of the SPF check. SPF may have false positives due to the sender's mail server settings being incorrect. (see above) Check your spam folder regularly to see if valid emails are moved there

You can choose to turn on the 'SpamAssassin' score if your email provider uses SpamAssassin. This does not run SpamAssassin on your iPhone/iPad and uses the SpamAssassin field in your email headers.
For advanced users, the 'Spam Score Required' will run some checks on your incoming email to see if it thinks it is spam. It is important that you read the section on the 'Spam Score' as some other emails that are not spam may be moved in error. View

Reject unknown BCC emails
This setting will filter out emails that you receive that do not have your email address in the "To" or "CC" address fields. Any email sender that sends you an email like this is send a BCC email to you. If you know the sender of the email then this filter will not apply. Many spam emails are sent in bulk and do not customise the "To" address.

Reject emails received from my email address with invalid SPF results
This setting will check for fake emails sent from your own email address. This setting will check SPF validation if your email server supports it.

Reject emails received from my email address with invalid DKIM signatures
This setting will check for fake emails sent from your own email address. This setting will check DKIM validation if your email server supports it.

History
You can check the history to see how an email was moved to the spam folder. The email rule and the filter used will be listed in the history screen. More