Certificates in the Trusted Certificate Store are used to validate signed emails and attachments. If an email or attachment displays a signing error because a certificate is missing, then you can add a certificate that you trust to the trusted store.

You can add your own certificates to a Trusted Certificate Store in 3 ways.

If you are viewing an email with a signing certificate then you can view the certificate list screen. View
Tap on the action menu button beside the certificate name and then select Trust Certificate from the action menu. This will add the selected certificate to the Trust Store.
Make sure you are sure the certificate is valid before you add it to the store.

You can view a certificate file in the Files screen or in a cloud server. When you view the certificate you can tap on the Trust Certificate button to add it to the trusted store.
You can import certificate files from your Mac or PC. Details

You can use the web browser inside AltaMail and download a certificate from a website. You can either download the certificate as a file or view it. Then you can tap on the Trust Certificate button to add it to the trusted store.