Add SMTP_HELLO_NAME to let operators set the EHLO/HELO identity

Google Workspace SMTP relay (smtp-relay.gmail.com) rejects generic
EHLO identifiers such as "localhost", which is Go net/smtp's default.
Operators can now set SMTP_HELLO_NAME to a valid hostname so that
c.Hello() is called before StartTLS, satisfying strict relay policies.

Fixes https://github.com/getprobo/probo/issues/1284

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/mailer/mailer.go

@@ -49,6 +49,7 @@ type (
 		User        string
 		Password    string
 		TLSRequired bool
+		HelloName   string
 	}
 
 	SendingWorkerOption func(*sendingHandler)
@@ -319,6 +320,12 @@ func (h *sendingHandler) sendMail(ctx context.Context, to []string, msg []byte)
 
 	defer func() { _ = c.Quit() }()
 
+	if h.smtp.HelloName != "" {
+		if err := c.Hello(h.smtp.HelloName); err != nil {
+			return fmt.Errorf("SMTP EHLO error: %w", err)
+		}
+	}
+
 	if h.smtp.TLSRequired {
 		if err := c.StartTLS(&tls.Config{ServerName: host}); err != nil {
 			return fmt.Errorf("TLS negotiation error: %w", err)