Refactor sendmail command to enhance recipient handling and error reporting

Dockerfile

@@ -6,21 +6,21 @@ RUN { \
     echo 'package main'; \
     echo ''; \
     echo 'import ('; \
-    echo '    "flag"'; \
     echo '    "fmt"'; \
     echo '    "io"'; \
+    echo '    "net/mail"'; \
     echo '    "net/smtp"'; \
     echo '    "os"'; \
+    echo '    "strings"'; \
     echo ')'; \
     echo ''; \
     echo 'func main() {'; \
-    echo '    from := flag.String("f", "minitor@localhost", "envelope sender")'; \
-    echo '    flag.Parse()'; \
-    echo '    recipients := flag.Args()'; \
-    echo ''; \
-    echo '    if len(recipients) == 0 {'; \
-    echo '        fmt.Fprintln(os.Stderr, "usage: sendmail [-f sender] recipient...")'; \
-    echo '        os.Exit(1)'; \
+    echo '    readHeaders := strings.Contains(strings.Join(os.Args[1:], " "), "-t")'; \
+    echo '    recipients := []string{}'; \
+    echo '    for _, arg := range os.Args[1:] {'; \
+    echo '        if !strings.HasPrefix(arg, "-") {'; \
+    echo '            recipients = append(recipients, arg)'; \
+    echo '        }'; \
     echo '    }'; \
     echo ''; \
     echo '    body, err := io.ReadAll(os.Stdin)'; \
@@ -29,6 +29,31 @@ RUN { \
     echo '        os.Exit(1)'; \
     echo '    }'; \
     echo ''; \
+    echo '    if readHeaders {'; \
+    echo '        msg, parseErr := mail.ReadMessage(strings.NewReader(string(body)))'; \
+    echo '        if parseErr != nil {'; \
+    echo '            fmt.Fprintln(os.Stderr, parseErr)'; \
+    echo '            os.Exit(1)'; \
+    echo '        }'; \
+    echo '        for _, hdr := range []string{"To", "Cc", "Bcc"} {'; \
+    echo '            if val := msg.Header.Get(hdr); val != "" {'; \
+    echo '                addrs, addrErr := mail.ParseAddressList(val)'; \
+    echo '                if addrErr != nil {'; \
+    echo '                    fmt.Fprintln(os.Stderr, addrErr)'; \
+    echo '                    os.Exit(1)'; \
+    echo '                }'; \
+    echo '                for _, addr := range addrs {'; \
+    echo '                    recipients = append(recipients, addr.Address)'; \
+    echo '                }'; \
+    echo '            }'; \
+    echo '        }'; \
+    echo '    }'; \
+    echo ''; \
+    echo '    if len(recipients) == 0 {'; \
+    echo '        fmt.Fprintln(os.Stderr, "usage: sendmail [-t] recipient...")'; \
+    echo '        os.Exit(1)'; \
+    echo '    }'; \
+    echo ''; \
     echo '    relay := os.Getenv("SMTP_RELAY")'; \
     echo '    if relay == "" {'; \
     echo '        relay = "172.17.0.2"'; \
@@ -39,9 +64,10 @@ RUN { \
     echo '        port = "25"'; \
     echo '    }'; \
     echo ''; \
-    echo '    sender := os.Getenv("SMTP_SENDER")'; \
+    echo '    sender := os.Getenv("EMAIL_FROM")'; \
     echo '    if sender == "" {'; \
-    echo '        sender = *from'; \
+    echo '        fmt.Fprintln(os.Stderr, "[sendmail] EMAIL_FROM is not set, skipping")'; \
+    echo '        os.Exit(0)'; \
     echo '    }'; \
     echo ''; \
     echo '    debug := os.Getenv("DEBUG") != ""'; \