add vendoring

vendor/gitlab.com/etke.cc/go/trysmtp/client.go

@@ -0,0 +1,82 @@
+package trysmtp
+
+import (
+	"crypto/tls"
+	"fmt"
+	"net"
+	"net/smtp"
+	"strings"
+)
+
+// SMTPAddrs priority list
+var SMTPAddrs = []string{":25", ":587", ":465"}
+
+// Connect to SMTP server and call MAIL and RCPT commands
+func Connect(from, to string) (*smtp.Client, error) {
+	localname := strings.SplitN(from, "@", 2)[1]
+	hostname := strings.SplitN(to, "@", 2)[1]
+	client, err := initClient(localname, hostname)
+	if err != nil {
+		return nil, err
+	}
+
+	err = client.Mail(from)
+	if err != nil {
+		client.Close()
+		return nil, err
+	}
+
+	err = client.Rcpt(to)
+	if err != nil {
+		client.Close()
+		return nil, err
+	}
+
+	return client, nil
+}
+
+func initClient(localname, hostname string) (*smtp.Client, error) {
+	mxs, err := net.LookupMX(hostname)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, mx := range mxs {
+		for _, addr := range SMTPAddrs {
+			client := trySMTP(localname, strings.TrimSuffix(mx.Host, "."), addr)
+			if client != nil {
+				return client, nil
+			}
+		}
+	}
+
+	// If there are no MX records, according to https://datatracker.ietf.org/doc/html/rfc5321#section-5.1,
+	// we're supposed to try talking directly to the host.
+	if len(mxs) == 0 {
+		for _, addr := range SMTPAddrs {
+			client := trySMTP(localname, hostname, addr)
+			if client != nil {
+				return client, nil
+			}
+		}
+	}
+
+	return nil, fmt.Errorf("target SMTP server not found")
+}
+
+func trySMTP(localname, mxhost, addr string) *smtp.Client {
+	conn, err := smtp.Dial(mxhost + addr)
+	if err != nil {
+		return nil
+	}
+	err = conn.Hello(localname)
+	if err != nil {
+		return nil
+	}
+	if ok, _ := conn.Extension("STARTTLS"); ok {
+		config := &tls.Config{ServerName: mxhost}
+		conn.StartTLS(config) //nolint:errcheck // if it doesn't work - we can't do anything anyway
+	}
+
+	return conn
+}