On January 22nd at around 5:00PM CT, Mailgun completed an update to the SSL certificates we use in order to correct numerous certificates that did not comply with the industry-developed CA/Browser Forum Baseline Requirements by Symantec’s PKI business. You can read more about this here - https://security.googleblog.com/2017/09/chromes-plan-to-distrust-symantec.html.
While this was intended to be a non-breaking change, we received multiple reports from clients utilizing Mailgun that they were receiving errors related to SSL. If you are receiving errors while attempting to send messages using Mailgun, you will need to update your local CA bundle. The steps for this vary depending on your OS, SMTP library or coding language that you are using and we recommend consulting the documentation for your application on how to update this locally.
If you are still unsure, please reach out to our support team for assistance.