SMTP Unable to Send Email over TLS port 465

There is a error on send any email via SMTP over TLS port 465. Unsecure over port 25 works. The same secrets work on other clients, thunderbird, fritzbox-router within TLS.
some logs over SMTP_DEBUG

[2022-01-06 17:06:06.335][vaultwarden::mail][DEBUG] SMTP lettre::transport::smtp::Error {
kind: Connection,
source: Failure(

Error {
code: ErrorCode(
cause: Some(
Error {
code: 337260938,
library: “SSL routines”,
function: “tls_process_ske_dhe”,
reason: “dh key too small”,
file: “…/ssl/statem/statem_clnt.c”,
line: 2150,

X509VerifyResult {
code: 18,
error: “self signed certificate”,

[2022-01-06 17:06:06.336][vaultwarden::mail][ERROR] SMTP Connection error: error:141A318A:SSL routines:tls_process_ske_dhe:dh key too small:…/ssl/statem/statem_clnt.c:2150: (self signed certificate)
[2022-01-06 17:06:06.337][response][INFO] POST /admin/test/smtp (test_smtp) => 400 Bad Request

I was renew the cert from the email-server, unfortunately no improvement.
Please help me to analyse and solve the issue.

Self signed certs could cause issues.
If it is not possible to generate a valid cert than either disable ssl/tls or disable certificate checking.

See vaultwarden/.env.template at main · dani-garcia/vaultwarden · GitHub