Gmail lets you Send emails from a different email address or alias so that If you use other email addresses or providers, you can send email from that address via Gmail.
To do this you configure an alias account in Gmail settings configured with the credentials of your mail service. This is useful if you want to consolidate email accounts or if you have a private email server and want to use Gmail to send email via this server using your various private email address domains.
At the start of April 2020 Google rolled out a security update that affected mail delivery using third party accounts. All emails sent via the alias account would not deliver, bouncing back with the error TLS Negotiation failed, the certificate doesn’t match the host.
TLS Negotiation failed, the certificate doesn’t match the host.
Google appears to be enforcing a new email encryption policy for secureTLS connections including validating that the host name on the mail server TLS certificate matches the canonical hostname (MX record) of the third party mail account.
If the host name does not match you can no longer use an encrypted TLS connection in Gmail to send email via your (or your ISP’s) mail servers.
For example, if your MX record resolves to mail.domain.com but the TLS certificate presented is for smtp.domain.com then Gmail will not connect to your mail server. For some users the only option to get mail working again is to revert to an unencrypted connection – strange that Google even allow that!
Google have updated the help article for Send emails from a different address or alias to include a section about this problem.
Google also no longer accept self signed certificates for TLS mail connections.
I use an EXIM4 docker container for my private mail relay, and use Gmail as the hub for my email send/receive. To workaround this problem I created a docker Certbot container and issued new LetsEncrypt TLS certificates for all my private mail domains used with Gmail as well as the primary TLS certificate for my Mail server.
I can confirm this resolves the problem and third party provider email sending via Gmail is now working again.
For anyone using Exim4 the way to configure Exim to use multiple TLS certificates is to dynamically match them to your mail domain, I did this using
Thousands of people have been affected by this. Considering the amount of people working from home or struggling to work at all during the Corona Virus pandemic its really bad timing by Google to implement new email security policies that are service affecting for a lot of users.