Email queue not sending emails after upgrade from Kentico 10 to 12

HAve you also applied the latest hotfix? The thing is that due to SMTP server authentication changes required by new SMTP providers, the authentication was changed and also Kentico started using MailKit package for this - and the SSL/TLS is now required. So, one option is to set your SMTP server to use a valid and trusted SSL certificate - MailKit FAQ. Or, do not apply one of the latest K12 hotfixes or, move to Kentico 13 where you can implement custom ISmtpClientFactory.

Thanks for replying, Juraj.

To answer your question, yes, I have the latest Kentico 12 hotfix version (v12.0.103). As for the SSL certificate on the SMTP server, we're using a wildcard certificate. Please let me know if that makes any difference.

I would try using different SMTP server for testing to see where the issue is, on what end. You can try using PaperCut or SMTP4Dev. Here are some other tips too.

I tested with smtp4dev both through the CMS and through a simple smtp client program I wrote for testing purposes. The smtp4dev server receives the emails from the custom app, but I receive the same error messages when I try to relay those messages to the on-premise smtp server. I also receive a different message when I set the smtp4dev test server as the default smtp server on Kentico 12.

"An error occurred while attempting to establish an SSL or TLS connection.

This usually means that the SSL certificate presented by the server is not trusted by the system for one or more of the following reasons:

1. The server is using a self-signed certificate which cannot be verified.
2. The local system is missing a Root or Intermediate certificate needed to verify the server's certificate.
3. A Certificate Authority CRL server for one or more of the certificates for one or more of the certificates in the chain is temporarily unavailable.
4. The certificate presented by the server is expired or invalid.
5. The set of SSL/TLS protocols supported by the client and server do not match.
6. You are trying to connect to a port that does not support SSL/TLS.

See https://github.com/jstedfast/mailkit/blob/master/FAQ.md#SslHandshakeException for possible solutions.

The handshake failed due to an unexpected packet format."

I will continue running some tests but I'll be looking into the server SSL certificates and other options. As always, thanks for your input. Any other suggestions would be helpful as well.

Marcelo

A quick update.

I tried a few examples from the Mailkit FAQ link, namely assigning the ServerCertificateValidationCallback function in the global.asax.cs file. I'm not sure if that is the best place to put the additional line of code, so if you know a better place to put it, please let me know.

The additional line of code allowed emails in the queue to be sent, but these emails are not appearing in the sent emails of the email queue. Emails from forms are not being sent though and that form functionality must be restored as soon as possible, so any suggestions on how to approach it would be most welcome.

I would recommend restoring backups and apply the last hotfix before the authentication change - which is 12.0.100. Otherwise, you need to do changes on the SMTP Server side and ensure proper SSL/TLS setup.

Thanks for letting me know. And for all the info provided as well.