upgrade deps; rewrite smtp session

vendor/github.com/jhillyerd/enmime/internal/coding/charsets.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"regexp"
 	"strings"
 
@@ -307,7 +306,7 @@ func ConvertToUTF8String(charset string, textBytes []byte) (string, error) {
 	}
 	input := bytes.NewReader(textBytes)
 	reader := transform.NewReader(input, csentry.e.NewDecoder())
-	output, err := ioutil.ReadAll(reader)
+	output, err := io.ReadAll(reader)
 	if err != nil {
 		return "", err
 	}

vendor/github.com/jhillyerd/enmime/internal/coding/headerext.go

@@ -54,20 +54,20 @@ func RFC2047Decode(s string) string {
 
 		default:
 			if decoded {
-				keyValuePair := strings.SplitAfter(s, "=")
-				if len(keyValuePair) < 2 {
+				key, value, found := strings.Cut(s, "=")
+				if !found {
 					return s
 				}
 
 				// Add quotes as needed.
-				if !strings.HasPrefix(keyValuePair[1], "\"") {
-					keyValuePair[1] = fmt.Sprintf("\"%s", keyValuePair[1])
+				if !strings.HasPrefix(value, "\"") {
+					value = fmt.Sprintf("\"%s", value)
 				}
-				if !strings.HasSuffix(keyValuePair[1], "\"") {
-					keyValuePair[1] = fmt.Sprintf("%s\"", keyValuePair[1])
+				if !strings.HasSuffix(value, "\"") {
+					value = fmt.Sprintf("%s\"", value)
 				}
 
-				return strings.Join(keyValuePair, "")
+				return fmt.Sprintf("%s=%s", key, value)
 			}
 
 			return s