--- sope-mime/NGMime/NGMimePartGenerator.m.orig	2019-12-17 21:34:06.000000000 +0100
+++ sope-mime/NGMime/NGMimePartGenerator.m	2020-10-02 09:42:47.030375229 +0200
@@ -258,6 +258,9 @@
     NSEnumerator *enumerator;
     BOOL         reset;
     
+    if ([headerFieldName isEqualToString:@"content-length"])
+      continue;
+
     if ([[_additionalHeaders objectsForKey:headerFieldName] count] > 0) {
       enumerator = [addHeaders objectEnumeratorForKey:headerFieldName];
       reset = YES;
@@ -274,6 +277,9 @@
   
   headerFieldNames = [addHeaders keyEnumerator];
   while ((headerFieldName = [headerFieldNames nextObject]) != nil) {
+    if ([headerFieldName isEqualToString:@"content-length"])
+      continue;
+
     [self appendHeaderField:headerFieldName
 	  values:[addHeaders objectEnumeratorForKey:headerFieldName]
 	  toData:data];