Use address literal as default EHLO argument

src/smtp/commands.rs

@@ -335,6 +335,7 @@ mod test {
     #[test]
     fn test_display() {
         let id = ClientId::Domain("localhost".to_string());
+        let id_ipv4 = ClientId::Ipv4(std::net::Ipv4Addr::new(127, 0, 0, 1));
         let email = EmailAddress::new("[email protected]".to_string()).unwrap();
         let mail_parameter = MailParameter::Other {
             keyword: "TEST".to_string(),
@@ -345,6 +346,10 @@ mod test {
             value: Some("value".to_string()),
         };
         assert_eq!(format!("{}", EhloCommand::new(id)), "EHLO localhost\r\n");
+        assert_eq!(
+            format!("{}", EhloCommand::new(id_ipv4)),
+            "EHLO [127.0.0.1]\r\n"
+        );
         assert_eq!(
             format!("{}", MailCommand::new(Some(email.clone()), vec![])),
             "MAIL FROM:<[email protected]>\r\n"

src/smtp/extension.rs

@@ -10,8 +10,12 @@ use std::fmt::{self, Display, Formatter};
 use std::net::{Ipv4Addr, Ipv6Addr};
 use std::result::Result;
 
-/// Default client id
-const DEFAULT_DOMAIN_CLIENT_ID: &str = "localhost";
+/// Default client id.
+///
+/// It passes
+/// `smtpd_helo_restrictions = reject_non_fqdn_helo_hostname`
+/// Postfix check, but not `reject_unknown_helo_hostname`.
+const DEFAULT_DOMAIN_CLIENT_ID: &str = "localhost.localdomain";
 
 /// Client identifier, the parameter to `EHLO`
 #[derive(PartialEq, Eq, Clone, Debug)]
@@ -28,12 +32,26 @@ pub enum ClientId {
     Ipv6(Ipv6Addr),
 }
 
+impl Default for ClientId {
+    fn default() -> Self {
+        // The most compatible address.
+        //
+        // It passes Postfix checks
+        // ```
+        // smtpd_helo_restrictions = reject_invalid_helo_hostname, reject_non_fqdn_helo_hostname, reject_unknown_helo_hostname
+        // smtpd_helo_required = yes
+        // smtpd_delay_reject = no
+        // ```
+        Self::Ipv4(Ipv4Addr::new(127, 0, 0, 1))
+    }
+}
+
 impl Display for ClientId {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         match *self {
             ClientId::Domain(ref value) => f.write_str(value),
-            ClientId::Ipv4(ref value) => write!(f, "{}", value),
-            ClientId::Ipv6(ref value) => write!(f, "{}", value),
+            ClientId::Ipv4(ref value) => write!(f, "[{}]", value),
+            ClientId::Ipv6(ref value) => write!(f, "[IPv6:{}]", value),
         }
     }
 }
@@ -44,8 +62,8 @@ impl ClientId {
         ClientId::Domain(domain)
     }
 
-    /// Defines a `ClientId` with the current hostname, of `localhost` if hostname could not be
-    /// found
+    /// Defines a `ClientId` with the current hostname, or
+    /// `localhost.localdomain` if hostname could not be found.
     pub fn hostname() -> ClientId {
         ClientId::Domain(get_hostname().unwrap_or_else(|| DEFAULT_DOMAIN_CLIENT_ID.to_string()))
     }

src/smtp/smtp_client.rs

@@ -108,7 +108,7 @@ impl SmtpClient {
                 smtp_utf8: false,
                 credentials: None,
                 connection_reuse: ConnectionReuseParameters::NoReuse,
-                hello_name: ClientId::new("localhost".to_string()),
+                hello_name: Default::default(),
                 authentication_mechanism: None,
                 force_set_auth: false,
                 timeout: Some(Duration::new(60, 0)),