Fix multipart header parsing in eml.go

The commit modifies the parseMultiPartHeader function to handle optional fields accurately. The delimiter was changed from "; " to ";" and whitespace is being trimmed from the start of optional fields to ensure correct splitting and mapping.

eml.go

@@ -142,7 +142,7 @@ func parseEMLHeaders(mailHeader *netmail.Header, msg *Msg) error {
 				addrStrings = append(addrStrings, addr.String())
 			}
 			if err = addrFunc(addrStrings...); err != nil {
-				return fmt.Errorf(`failed to parse %q header: %w`, HeaderTo, err)
+				return fmt.Errorf(`failed to parse %q header: %w`, addrHeader, err)
 			}
 		}
 	}
@@ -372,7 +372,8 @@ func parseMultiPartHeader(multiPartHeader string) (header string, optional map[s
 	headerSplit := strings.SplitN(multiPartHeader, ";", 2)
 	header = headerSplit[0]
 	if len(headerSplit) == 2 {
-		optSplit := strings.SplitN(headerSplit[1], "=", 2)
+		optString := strings.TrimLeft(headerSplit[1], " ")
+		optSplit := strings.SplitN(optString, "=", 2)
 		if len(optSplit) == 2 {
 			optional[optSplit[0]] = optSplit[1]
 		}