diff --git a/docs/libcurl/curl_mime_init.3 b/docs/libcurl/curl_mime_init.3
index 9ee466f12..09c3ee166 100644
--- a/docs/libcurl/curl_mime_init.3
+++ b/docs/libcurl/curl_mime_init.3
@@ -31,11 +31,14 @@ curl_mime_init - create a mime handle
 curl_mime *curl_mime_init(CURL *easy_handle);
 .fi
 .SH DESCRIPTION
-\fIcurl_mime_init(3)\fP creates a handle to a new empty mime structure
-intended to be used with \fIeasy_handle\fP. This mime structure can be
-subsequently filled using the mime API, then attached to \fIeasy_handle\fP
-using option \fICURLOPT_MIMEPOST(3)\fP within a \fIcurl_easy_setopt(3)\fP
-call.
+\fIcurl_mime_init(3)\fP creates a handle to a new empty mime structure,
+This mime structure can be subsequently filled using the mime API, then
+attached to some easy handle using option \fICURLOPT_MIMEPOST(3)\fP within
+a \fIcurl_easy_setopt(3)\fP call or added as a multipart in another mime
+handle's part using \fIcurl_mime_subparts(3)\fP.
+
+\fIeasy_handle\fP is used for part separator randomization and error
+reporting. It does not need to be the final target handle.
 
 Using a mime handle is the recommended way to post an HTTP form, format and
 send a multi-part email with SMTP or upload such an email to an IMAP server.
@@ -65,5 +68,6 @@ As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.
 A mime struct handle, or NULL upon failure.
 .SH "SEE ALSO"
 .BR curl_mime_addpart "(3),"
+.BR curl_mime_subparts "(3),"
 .BR curl_mime_free "(3),"
 .BR CURLOPT_MIMEPOST "(3)"
diff --git a/lib/easy.c b/lib/easy.c
index b61ea7b7c..d7f93be1e 100644
--- a/lib/easy.c
+++ b/lib/easy.c
@@ -828,7 +828,7 @@ static CURLcode dupset(struct Curl_easy *dst, struct Curl_easy *src)
   /* Copy src->set into dst->set first, then deal with the strings
      afterwards */
   dst->set = src->set;
-  Curl_mime_initpart(&dst->set.mimepost, dst);
+  Curl_mime_initpart(&dst->set.mimepost);
 
   /* clear all string pointers first */
   memset(dst->set.str, 0, STRING_LAST * sizeof(char *));
@@ -862,7 +862,7 @@ static CURLcode dupset(struct Curl_easy *dst, struct Curl_easy *src)
   }
 
   /* Duplicate mime data. */
-  result = Curl_mime_duppart(&dst->set.mimepost, &src->set.mimepost);
+  result = Curl_mime_duppart(dst, &dst->set.mimepost, &src->set.mimepost);
 
   if(src->set.resolve)
     dst->state.resolve = dst->set.resolve;
diff --git a/lib/formdata.c b/lib/formdata.c
index 058e06f1f..b30e8deee 100644
--- a/lib/formdata.c
+++ b/lib/formdata.c
@@ -715,10 +715,10 @@ int curl_formget(struct curl_httppost *form, void *arg,
   CURLcode result;
   curl_mimepart toppart;
 
-  Curl_mime_initpart(&toppart, NULL); /* default form is empty */
+  Curl_mime_initpart(&toppart); /* default form is empty */
   result = Curl_getformdata(NULL, &toppart, form, NULL);
   if(!result)
-    result = Curl_mime_prepare_headers(&toppart, "multipart/form-data",
+    result = Curl_mime_prepare_headers(NULL, &toppart, "multipart/form-data",
                                        NULL, MIMESTRATEGY_FORM);
 
   while(!result) {
diff --git a/lib/http.c b/lib/http.c
index 66d3b4dd6..85528a221 100644
--- a/lib/http.c
+++ b/lib/http.c
@@ -265,7 +265,7 @@ static CURLcode http_setup_conn(struct Curl_easy *data,
   if(!http)
     return CURLE_OUT_OF_MEMORY;
 
-  Curl_mime_initpart(&http->form, data);
+  Curl_mime_initpart(&http->form);
   data->req.p.http = http;
 
   if(data->state.httpwant == CURL_HTTP_VERSION_3) {
@@ -2303,7 +2303,7 @@ CURLcode Curl_http_body(struct Curl_easy *data, struct connectdata *conn,
       cthdr = "multipart/form-data";
 
     curl_mime_headers(http->sendit, data->set.headers, 0);
-    result = Curl_mime_prepare_headers(http->sendit, cthdr,
+    result = Curl_mime_prepare_headers(data, http->sendit, cthdr,
                                        NULL, MIMESTRATEGY_FORM);
     curl_mime_headers(http->sendit, NULL, 0);
     if(!result)
diff --git a/lib/imap.c b/lib/imap.c
index 03dc19166..a462ff042 100644
--- a/lib/imap.c
+++ b/lib/imap.c
@@ -777,7 +777,7 @@ static CURLcode imap_perform_append(struct Curl_easy *data)
 
     /* Add external headers and mime version. */
     curl_mime_headers(&data->set.mimepost, data->set.headers, 0);
-    result = Curl_mime_prepare_headers(&data->set.mimepost, NULL,
+    result = Curl_mime_prepare_headers(data, &data->set.mimepost, NULL,
                                        NULL, MIMESTRATEGY_MAIL);
 
     if(!result)
diff --git a/lib/mime.c b/lib/mime.c
index 117c66f4d..e3f2821db 100644
--- a/lib/mime.c
+++ b/lib/mime.c
@@ -1175,7 +1175,7 @@ void Curl_mime_cleanpart(curl_mimepart *part)
   Curl_safefree(part->mimetype);
   Curl_safefree(part->name);
   Curl_safefree(part->filename);
-  Curl_mime_initpart(part, part->easy);
+  Curl_mime_initpart(part);
 }
 
 /* Recursively delete a mime handle and its parts. */
@@ -1195,7 +1195,8 @@ void curl_mime_free(curl_mime *mime)
   }
 }
 
-CURLcode Curl_mime_duppart(curl_mimepart *dst, const curl_mimepart *src)
+CURLcode Curl_mime_duppart(struct Curl_easy *data,
+                           curl_mimepart *dst, const curl_mimepart *src)
 {
   curl_mime *mime;
   curl_mimepart *d;
@@ -1224,13 +1225,13 @@ CURLcode Curl_mime_duppart(curl_mimepart *dst, const curl_mimepart *src)
   case MIMEKIND_MULTIPART:
     /* No one knows about the cloned subparts, thus always attach ownership
        to the part. */
-    mime = curl_mime_init(dst->easy);
+    mime = curl_mime_init(data);
     res = mime? curl_mime_subparts(dst, mime): CURLE_OUT_OF_MEMORY;
 
     /* Duplicate subparts. */
     for(s = ((curl_mime *) src->arg)->firstpart; !res && s; s = s->nextpart) {
       d = curl_mime_addpart(mime);
-      res = d? Curl_mime_duppart(d, s): CURLE_OUT_OF_MEMORY;
+      res = d? Curl_mime_duppart(data, d, s): CURLE_OUT_OF_MEMORY;
     }
     break;
   default:  /* Invalid kind: should not occur. */
@@ -1282,7 +1283,6 @@ curl_mime *curl_mime_init(struct Curl_easy *easy)
   mime = (curl_mime *) malloc(sizeof(*mime));
 
   if(mime) {
-    mime->easy = easy;
     mime->parent = NULL;
     mime->firstpart = NULL;
     mime->lastpart = NULL;
@@ -1302,10 +1302,9 @@ curl_mime *curl_mime_init(struct Curl_easy *easy)
 }
 
 /* Initialize a mime part. */
-void Curl_mime_initpart(curl_mimepart *part, struct Curl_easy *easy)
+void Curl_mime_initpart(curl_mimepart *part)
 {
   memset((char *) part, 0, sizeof(*part));
-  part->easy = easy;
   part->lastreadstatus = 1; /* Successful read status. */
   mimesetstate(&part->state, MIMESTATE_BEGIN, NULL);
 }
@@ -1321,7 +1320,7 @@ curl_mimepart *curl_mime_addpart(curl_mime *mime)
   part = (curl_mimepart *) malloc(sizeof(*part));
 
   if(part) {
-    Curl_mime_initpart(part, mime->easy);
+    Curl_mime_initpart(part);
     part->parent = mime;
 
     if(mime->lastpart)
@@ -1551,10 +1550,6 @@ CURLcode Curl_mime_set_subparts(curl_mimepart *part,
   cleanup_part_content(part);
 
   if(subparts) {
-    /* Must belong to the same data handle. */
-    if(part->easy && subparts->easy && part->easy != subparts->easy)
-      return CURLE_BAD_FUNCTION_ARGUMENT;
-
     /* Should not have been attached already. */
     if(subparts->parent)
       return CURLE_BAD_FUNCTION_ARGUMENT;
@@ -1565,8 +1560,7 @@ CURLcode Curl_mime_set_subparts(curl_mimepart *part,
       while(root->parent && root->parent->parent)
         root = root->parent->parent;
       if(subparts == root) {
-        if(part->easy)
-          failf(part->easy, "Can't add itself as a subpart");
+        /* Can't add as a subpart of itself. */
         return CURLE_BAD_FUNCTION_ARGUMENT;
       }
     }
@@ -1766,7 +1760,8 @@ static bool content_type_match(const char *contenttype,
   return FALSE;
 }
 
-CURLcode Curl_mime_prepare_headers(curl_mimepart *part,
+CURLcode Curl_mime_prepare_headers(struct Curl_easy *data,
+                                   curl_mimepart *part,
                                    const char *contenttype,
                                    const char *disposition,
                                    enum mimestrategy strategy)
@@ -1835,12 +1830,12 @@ CURLcode Curl_mime_prepare_headers(curl_mimepart *part,
       char *filename = NULL;
 
       if(part->name) {
-        name = escape_string(part->easy, part->name, strategy);
+        name = escape_string(data, part->name, strategy);
         if(!name)
           ret = CURLE_OUT_OF_MEMORY;
       }
       if(!ret && part->filename) {
-        filename = escape_string(part->easy, part->filename, strategy);
+        filename = escape_string(data, part->filename, strategy);
         if(!filename)
           ret = CURLE_OUT_OF_MEMORY;
       }
@@ -1897,7 +1892,8 @@ CURLcode Curl_mime_prepare_headers(curl_mimepart *part,
     if(content_type_match(contenttype, STRCONST("multipart/form-data")))
       disposition = "form-data";
     for(subpart = mime->firstpart; subpart; subpart = subpart->nextpart) {
-      ret = Curl_mime_prepare_headers(subpart, NULL, disposition, strategy);
+      ret = Curl_mime_prepare_headers(data, subpart, NULL,
+                                      disposition, strategy);
       if(ret)
         return ret;
     }
diff --git a/lib/mime.h b/lib/mime.h
index bafde29f4..b9ea0f101 100644
--- a/lib/mime.h
+++ b/lib/mime.h
@@ -99,7 +99,6 @@ struct mime_state {
 
 /* A mime multipart. */
 struct curl_mime {
-  struct Curl_easy *easy;          /* The associated easy handle. */
   curl_mimepart *parent;           /* Parent part. */
   curl_mimepart *firstpart;        /* First part. */
   curl_mimepart *lastpart;         /* Last part. */
@@ -109,7 +108,6 @@ struct curl_mime {
 
 /* A mime part. */
 struct curl_mimepart {
-  struct Curl_easy *easy;          /* The associated easy handle. */
   curl_mime *parent;               /* Parent mime structure. */
   curl_mimepart *nextpart;         /* Forward linked list. */
   enum mimekind kind;              /* The part kind. */
@@ -139,14 +137,16 @@ CURLcode Curl_mime_add_header(struct curl_slist **slp, const char *fmt, ...);
                                     !defined(CURL_DISABLE_IMAP))
 
 /* Prototypes. */
-void Curl_mime_initpart(struct curl_mimepart *part, struct Curl_easy *easy);
+void Curl_mime_initpart(struct curl_mimepart *part);
 void Curl_mime_cleanpart(struct curl_mimepart *part);
-CURLcode Curl_mime_duppart(struct curl_mimepart *dst,
+CURLcode Curl_mime_duppart(struct Curl_easy *data,
+                           struct curl_mimepart *dst,
                            const curl_mimepart *src);
 CURLcode Curl_mime_set_subparts(struct curl_mimepart *part,
                                 struct curl_mime *subparts,
                                 int take_ownership);
-CURLcode Curl_mime_prepare_headers(struct curl_mimepart *part,
+CURLcode Curl_mime_prepare_headers(struct Curl_easy *data,
+                                   struct curl_mimepart *part,
                                    const char *contenttype,
                                    const char *disposition,
                                    enum mimestrategy strategy);
@@ -159,11 +159,11 @@ void Curl_mime_unpause(struct curl_mimepart *part);
 
 #else
 /* if disabled */
-#define Curl_mime_initpart(x,y)
+#define Curl_mime_initpart(x)
 #define Curl_mime_cleanpart(x)
-#define Curl_mime_duppart(x,y) CURLE_OK /* Nothing to duplicate. Succeed */
+#define Curl_mime_duppart(x,y,z) CURLE_OK /* Nothing to duplicate. Succeed */
 #define Curl_mime_set_subparts(a,b,c) CURLE_NOT_BUILT_IN
-#define Curl_mime_prepare_headers(a,b,c,d) CURLE_NOT_BUILT_IN
+#define Curl_mime_prepare_headers(a,b,c,d,e) CURLE_NOT_BUILT_IN
 #define Curl_mime_size(x) (curl_off_t) -1
 #define Curl_mime_read NULL
 #define Curl_mime_rewind(x) ((void)x, CURLE_NOT_BUILT_IN)
diff --git a/lib/smtp.c b/lib/smtp.c
index cbaf482e9..085faf9e3 100644
--- a/lib/smtp.c
+++ b/lib/smtp.c
@@ -697,7 +697,7 @@ static CURLcode smtp_perform_mail(struct Curl_easy *data)
 
     /* Add external headers and mime version. */
     curl_mime_headers(&data->set.mimepost, data->set.headers, 0);
-    result = Curl_mime_prepare_headers(&data->set.mimepost, NULL,
+    result = Curl_mime_prepare_headers(data, &data->set.mimepost, NULL,
                                        NULL, MIMESTRATEGY_MAIL);
 
     if(!result)
diff --git a/lib/url.c b/lib/url.c
index 74201f9bb..0af812bcd 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -560,7 +560,7 @@ CURLcode Curl_init_userdefined(struct Curl_easy *data)
   /* make libcurl quiet by default: */
   set->hide_progress = TRUE;  /* CURLOPT_NOPROGRESS changes these */
 
-  Curl_mime_initpart(&set->mimepost, data);
+  Curl_mime_initpart(&set->mimepost);
 
   /*
    * libcurl 7.10 introduced SSL verification *by default*! This needs to be