Rust Lettre command not recognized - starttls

Question

I try to send email from my backend using lettre to my mail server but I get the error

lettre::transport::smtp::Error { kind: Permanent(Code { severity: PermanentNegativeCompletion, category: Syntax, detail: Zero }), source: "5.5.2 Error: command not recognized" }

Postfix side

improper command pipelining after EHLO from x.x.x.x: QUIT\r\n

The following implementation comes from the examples provided here.

use lettre::{
    message::header::ContentType, transport::smtp::authentication::Credentials, Message,
    SmtpTransport, Transport,
};

fn main() {
    let email = Message::builder()
        .from("[email protected]".parse().unwrap())
        .to("[email protected]".parse().unwrap())
        .subject("Test mailer")
        .header(ContentType::TEXT_PLAIN)
        .body(String::from("Hello world !"))
        .unwrap();

    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());

    let mailer = SmtpTransport::starttls_relay("mydomain.tld")
        .unwrap()
        .credentials(creds)
        .build();

    // Send the email
    match mailer.send(&email) {
        Ok(_) => println!("Email sent successfully!"),
        Err(e) => panic!("Could not send email: {e:?}"),
    }
}

I only have issues with this script, I have no problems with a dummy python script using smptlib or mail clients.

UPDATE : By using the internal ip of the postfix server (containerized) instead of the domain name, I get the following error. This issue only happens with lettre.

Some(lettre::transport::smtp::Error { kind: Connection, source: Failure(Ssl(Error { code: ErrorCode(1), cause: Some(Ssl(ErrorStack([Error { code: 337047686, library: "SSL routines", function: "tls_process_server_certificate", reason: "certificate verify failed", file: "ssl/statem/statem_clnt.c", line: 1919 }]))) }, X509VerifyResult { code: 64, error: "IP address mismatch" })) })