You can view emails digitally signed with certificates.

For a signed email, the signing status is displayed above the attachments list. The status can be valid or display a certificate error. You can tap on the digital signed status message to view all of the certificates used in digital signing of the email.

When you view the digital signing certificate list, you can view the details of each certificate with the action menu to the right of each certificate name.
If the certificate status is invalid then you can add it to your Trusted Certificate Store. Tap on Trust Certificate in the action menu if you know the certificate is valid. More

When email attachments are digitally signed you will see a status symbol beside the attachment name. If the attachment digital signing is valid then a yellow star icon will be displayed.
If the attachment has a digital signing error then a red alert icon is displayed. Attachments that do not have a valid digital signing status will show as type "p7m" until they are valid.
Tap on the signing status symbol to view all of the digital signing certificates for the attachment. If the digital signing is invalid then you can tap on the Ignore Security Error button to view the attachment even if it has errors.
You can also tap on Trust Certificate in the action menu for a certificate if you know the certificate is valid. More. This will validate the attachment and any other attachment with the same trusted certificate for viewing next time.