Traefik doesn't use SANs in certificate?

When I try to send a mail via SMTP with a go app, it throws this error: x509: certificate relies on legacy Common Name field, use SANs or temporarily enable Common Name matching with GODEBUG=x509ignoreCN=0.

It seems that SANs are enabled by default. I added this label "traefik.frontend.passTLSClientCert.infos.issuer.commonName=false" to my traefik container so that commonName is disabled.

I've disabled tls in Mailcow. TLS is handled by traefik. So I assume its related to traefik. Could that be an issue with Traefik?