When Mail (iOS 10.3.3) attempts to access my mail host mail.yourdomain.*, a "Not Trusted" certificate error is shown.
If you are using iPhone, follow these steps:
Open 'Settings > Mail > Accounts', tap the account and address, and then re-enter the hostname. If this doesn't work, try setting up email as a new account. Go to 'Settings > Mail > Accounts > Add Account'. If the message 'Cannot Verify Server Identity' appears, tap the 'Details' option below that message, and then tap the Trust button in the top-right corner.
By clicking on the details for the specific certificate, you will be able to trust it from there.