Recently, our app started returning the following error:
Could not create SSL/TLS secure channel
This is being caused by an older version of the TLS protocol, which MailChimp will no longer allow.
For anyone searching for this, I put this post out here to help any other developers trying to troubleshoot this. There are a variety of results you’ll find when searching for this error, but it mainly comes down to the version of .NET your application targets. The thing is though, that alone, isn’t the issue — e.g. your application needs to be targeting .NET version 4.5 or above in order to utilize the latest TLS protocol.
Your application could already be targeting .NET version 4.5, or even 4.7.2, and you’ll still get this error.
You have to explicitly specify the TLS protocol version before making your connection to MailChimp:
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12
Once that’s set, you should be able to successfully connect to MailChimp again. 🙂