@Martin_Forster , it sounds like SSL option in credentials is redundant. To be honest, I’m surprised to see it there in the first place but not surprised it gets ignored. The control of the protocol used (secure vs not secure) in establishing the connection to the server is rightfully controlled by the Email node itself.
In your case, it looks like the SMTP server does not have a valid SSL certificate but Email node establishes connection via secure protocol by default. Since certificate is invalid the response let the client know of that. Ignoring the warning works as your SMTP allows insecure connections.