Visit https://<tenant-domain>/admin/custom_ca to configure a certificate authority for use by all of your IMAP and HTTP Request actions. Enter the PEM encoded X.509 public certificate (or certificate chain) for your custom certificate authority and hit save. Alternatively, you can also create and manage dedicated custom certificate authorities from the same UI. See our What's New post here.
Your custom certificate authority will then be used in addition to the standard public certificate authorities when validating the certificates of the IMAP and HTTP endpoints your actions are contacting.