We're using a form that automatically creates a lead in Salesforce after which an email to the person filling out the form is sent automatically as well. This works well for just about everything but emails are not sent out to users that have an email address from the "web.de" and "gmx.de" domains.
Specifically, the lead shows that the email was sent out without any error, but when requesting emails logs from Salesforce, each address from those domains shows the exact same problem:
451 4.7.5 [internal] TLS negotiation failed
The problem seems to be limited to those two domains, but it's still very annoying that this happens (and that I can't even see this happened in the email activity of the lead. Does anybody understand why this error occurs for these two domains specifically and what can be done to fix this? I'll take this to the support organisation for these two providers (if it's not the same one) too, but I thought perhaps some Salesforce expert has run into this before with those domains or others so that we can learn from that.