got first record does not look like a TLS handshake when trying to connect to mailtrap smtp

I have following code for creating an SMTP connection:

func NewSMTPClient(host, username, password string, port int) (*smtp.Client, error) {
    tlsConfig := &tls.Config{ServerName: host}

    conn, err := tls.Dial("tcp", fmt.Sprintf("%s:%d", host, port), tlsConfig)
    if err != nil {
        return nil, fmt.Errorf("smtp client: tcp dial: %s", err.Error())
    }

    c, err := smtp.NewClient(conn, host)
    if err != nil {
        return nil, fmt.Errorf("smtp client: new client: %s", err.Error())
    }

    auth := smtp.PlainAuth("", username, password, host)
    if err = c.Auth(auth); err != nil {
        return nil, fmt.Errorf("smtp client: get auth: %s", err.Error())
    }

    return &c, nil
}

This code works very well with Yandex SMTP server. But when I tried to test it with MailTrap in my unit tests (since I didn't want to actually send email to people every time I test the code), I got this error:

smtp client: tcp dial: tls: first record does not look like a TLS handshake

EDIT 1:

I have tried to set InsecureSkipVerify to true, but still got the same error; though the MailTrap configuration says that I should be able to use TLS:

TLS: Optional (STARTTLS on all ports)

Probably the SMTP server you are trying to connect to doesn't support STARTTLS.