diff --git a/src/Makefile.am b/src/Makefile.am
index ad8552b..f1eb996 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -7,7 +7,7 @@ mbsync_SOURCES = \
 	driver.c drv_proxy.c \
 	drv_imap.c imap_msgs.c \
 	drv_maildir.c \
-	sync.c sync_state.c \
+	sync.c sync_state.c sync_msg_cvt.c \
 	main.c main_sync.c main_list.c
 noinst_HEADERS = \
 	common.h config.h socket.h \
diff --git a/src/sync.c b/src/sync.c
index dd85d7e..f076241 100644
--- a/src/sync.c
+++ b/src/sync.c
@@ -79,15 +79,6 @@ sanitize_flags( uchar tflags, sync_vars_t *svars, int t )
 }
 
 
-typedef struct copy_vars {
-	void (*cb)( int sts, uint uid, struct copy_vars *vars );
-	void *aux;
-	sync_rec_t *srec; /* also ->tuid */
-	message_t *msg;
-	msg_data_t data;
-	int minimal;
-} copy_vars_t;
-
 static void msg_fetched( int sts, void *aux );
 
 static void
@@ -103,177 +94,6 @@ copy_msg( copy_vars_t *vars )
 
 static void msg_stored( int sts, uint uid, void *aux );
 
-static void
-copy_msg_bytes( char **out_ptr, const char *in_buf, uint *in_idx, uint in_len, int in_cr, int out_cr )
-{
-	char *out = *out_ptr;
-	uint idx = *in_idx;
-	if (out_cr != in_cr) {
-		char c;
-		if (out_cr) {
-			for (; idx < in_len; idx++) {
-				if ((c = in_buf[idx]) != '\r') {
-					if (c == '\n')
-						*out++ = '\r';
-					*out++ = c;
-				}
-			}
-		} else {
-			for (; idx < in_len; idx++) {
-				if ((c = in_buf[idx]) != '\r')
-					*out++ = c;
-			}
-		}
-	} else {
-		memcpy( out, in_buf + idx, in_len - idx );
-		out += in_len - idx;
-		idx = in_len;
-	}
-	*out_ptr = out;
-	*in_idx = idx;
-}
-
-static char *
-copy_msg_convert( int in_cr, int out_cr, copy_vars_t *vars )
-{
-	char *in_buf = vars->data.data;
-	uint in_len = vars->data.len;
-	uint idx = 0, sbreak = 0, ebreak = 0, break2 = UINT_MAX;
-	uint lines = 0, hdr_crs = 0, bdy_crs = 0, app_cr = 0, extra = 0;
-	uint add_subj = 0;
-
-	if (vars->srec) {
-	  nloop: ;
-		uint start = idx;
-		uint line_crs = 0;
-		while (idx < in_len) {
-			char c = in_buf[idx++];
-			if (c == '\r') {
-				line_crs++;
-			} else if (c == '\n') {
-				if (!ebreak && starts_with_upper( in_buf + start, (int)(in_len - start), "X-TUID: ", 8 )) {
-					extra = (sbreak = start) - (ebreak = idx);
-					if (!vars->minimal)
-						goto oke;
-				} else {
-					if (break2 == UINT_MAX && vars->minimal &&
-					    starts_with_upper( in_buf + start, (int)(in_len - start), "SUBJECT:", 8 )) {
-						break2 = start + 8;
-						if (break2 < in_len && in_buf[break2] == ' ')
-							break2++;
-					}
-					lines++;
-					hdr_crs += line_crs;
-				}
-				if (idx - line_crs - 1 == start) {
-					if (!ebreak)
-						sbreak = ebreak = start;
-					if (vars->minimal) {
-						in_len = idx;
-						if (break2 == UINT_MAX) {
-							break2 = start;
-							add_subj = 1;
-						}
-					}
-					goto oke;
-				}
-				goto nloop;
-			}
-		}
-		free( in_buf );
-		return "has incomplete header";
-	  oke:
-		app_cr = out_cr && (!in_cr || hdr_crs);
-		extra += 8 + TUIDL + app_cr + 1;
-	}
-	if (out_cr != in_cr) {
-		for (; idx < in_len; idx++) {
-			char c = in_buf[idx];
-			if (c == '\r')
-				bdy_crs++;
-			else if (c == '\n')
-				lines++;
-		}
-		extra -= hdr_crs + bdy_crs;
-		if (out_cr)
-			extra += lines;
-	}
-
-	uint dummy_msg_len = 0;
-	char dummy_msg_buf[256];
-	static const char dummy_pfx[] = "[placeholder] ";
-	static const char dummy_subj[] = "Subject: [placeholder] (No Subject)";
-	static const char dummy_msg[] =
-		"Having a size of %s, this message is over the MaxSize limit.%s"
-		"Flag it and sync again (Sync mode Upgrade) to fetch its real contents.%s";
-	static const char dummy_flag[] =
-		"%s"
-		"The original message is flagged as important.%s";
-
-	if (vars->minimal) {
-		char sz[32];
-
-		if (vars->msg->size < 1024000)
-			sprintf( sz, "%dKiB", (int)(vars->msg->size >> 10) );
-		else
-			sprintf( sz, "%.1fMiB", vars->msg->size / 1048576. );
-		const char *nl = app_cr ? "\r\n" : "\n";
-		dummy_msg_len = (uint)sprintf( dummy_msg_buf, dummy_msg, sz, nl, nl );
-		if (vars->data.flags & F_FLAGGED) {
-			vars->data.flags &= ~F_FLAGGED;
-			dummy_msg_len += (uint)sprintf( dummy_msg_buf + dummy_msg_len, dummy_flag, nl, nl );
-		}
-		extra += dummy_msg_len;
-		extra += add_subj ? strlen(dummy_subj) + app_cr + 1 : strlen(dummy_pfx);
-	}
-
-	vars->data.len = in_len + extra;
-	if (vars->data.len > INT_MAX) {
-		free( in_buf );
-		return "is too big after conversion";
-	}
-	char *out_buf = vars->data.data = nfmalloc( vars->data.len );
-	idx = 0;
-	if (vars->srec) {
-		if (break2 < sbreak) {
-			copy_msg_bytes( &out_buf, in_buf, &idx, break2, in_cr, out_cr );
-			memcpy( out_buf, dummy_pfx, strlen(dummy_pfx) );
-			out_buf += strlen(dummy_pfx);
-		}
-		copy_msg_bytes( &out_buf, in_buf, &idx, sbreak, in_cr, out_cr );
-
-		memcpy( out_buf, "X-TUID: ", 8 );
-		out_buf += 8;
-		memcpy( out_buf, vars->srec->tuid, TUIDL );
-		out_buf += TUIDL;
-		if (app_cr)
-			*out_buf++ = '\r';
-		*out_buf++ = '\n';
-		idx = ebreak;
-
-		if (break2 != UINT_MAX && break2 >= sbreak) {
-			copy_msg_bytes( &out_buf, in_buf, &idx, break2, in_cr, out_cr );
-			if (!add_subj) {
-				memcpy( out_buf, dummy_pfx, strlen(dummy_pfx) );
-				out_buf += strlen(dummy_pfx);
-			} else {
-				memcpy( out_buf, dummy_subj, strlen(dummy_subj) );
-				out_buf += strlen(dummy_subj);
-				if (app_cr)
-					*out_buf++ = '\r';
-				*out_buf++ = '\n';
-			}
-		}
-	}
-	copy_msg_bytes( &out_buf, in_buf, &idx, in_len, in_cr, out_cr );
-
-	if (vars->minimal)
-		memcpy( out_buf, dummy_msg_buf, dummy_msg_len );
-
-	free( in_buf );
-	return NULL;
-}
-
 static void
 msg_fetched( int sts, void *aux )
 {
diff --git a/src/sync_msg_cvt.c b/src/sync_msg_cvt.c
new file mode 100644
index 0000000..4f8403e
--- /dev/null
+++ b/src/sync_msg_cvt.c
@@ -0,0 +1,179 @@
+// SPDX-FileCopyrightText: 2000-2002 Michael R. Elkins <me@mutt.org>
+// SPDX-FileCopyrightText: 2002-2022 Oswald Buddenhagen <ossi@users.sf.net>
+// SPDX-License-Identifier: GPL-2.0-or-later WITH LicenseRef-isync-GPL-exception
+//
+// mbsync - mailbox synchronizer
+//
+
+#include "sync_p.h"
+
+static void
+copy_msg_bytes( char **out_ptr, const char *in_buf, uint *in_idx, uint in_len, int in_cr, int out_cr )
+{
+	char *out = *out_ptr;
+	uint idx = *in_idx;
+	if (out_cr != in_cr) {
+		char c;
+		if (out_cr) {
+			for (; idx < in_len; idx++) {
+				if ((c = in_buf[idx]) != '\r') {
+					if (c == '\n')
+						*out++ = '\r';
+					*out++ = c;
+				}
+			}
+		} else {
+			for (; idx < in_len; idx++) {
+				if ((c = in_buf[idx]) != '\r')
+					*out++ = c;
+			}
+		}
+	} else {
+		memcpy( out, in_buf + idx, in_len - idx );
+		out += in_len - idx;
+		idx = in_len;
+	}
+	*out_ptr = out;
+	*in_idx = idx;
+}
+
+char *
+copy_msg_convert( int in_cr, int out_cr, copy_vars_t *vars )
+{
+	char *in_buf = vars->data.data;
+	uint in_len = vars->data.len;
+	uint idx = 0, sbreak = 0, ebreak = 0, break2 = UINT_MAX;
+	uint lines = 0, hdr_crs = 0, bdy_crs = 0, app_cr = 0, extra = 0;
+	uint add_subj = 0;
+
+	if (vars->srec) {
+	  nloop: ;
+		uint start = idx;
+		uint line_crs = 0;
+		while (idx < in_len) {
+			char c = in_buf[idx++];
+			if (c == '\r') {
+				line_crs++;
+			} else if (c == '\n') {
+				if (!ebreak && starts_with_upper( in_buf + start, (int)(in_len - start), "X-TUID: ", 8 )) {
+					extra = (sbreak = start) - (ebreak = idx);
+					if (!vars->minimal)
+						goto oke;
+				} else {
+					if (break2 == UINT_MAX && vars->minimal &&
+					    starts_with_upper( in_buf + start, (int)(in_len - start), "SUBJECT:", 8 )) {
+						break2 = start + 8;
+						if (break2 < in_len && in_buf[break2] == ' ')
+							break2++;
+					}
+					lines++;
+					hdr_crs += line_crs;
+				}
+				if (idx - line_crs - 1 == start) {
+					if (!ebreak)
+						sbreak = ebreak = start;
+					if (vars->minimal) {
+						in_len = idx;
+						if (break2 == UINT_MAX) {
+							break2 = start;
+							add_subj = 1;
+						}
+					}
+					goto oke;
+				}
+				goto nloop;
+			}
+		}
+		free( in_buf );
+		return "has incomplete header";
+	  oke:
+		app_cr = out_cr && (!in_cr || hdr_crs);
+		extra += 8 + TUIDL + app_cr + 1;
+	}
+	if (out_cr != in_cr) {
+		for (; idx < in_len; idx++) {
+			char c = in_buf[idx];
+			if (c == '\r')
+				bdy_crs++;
+			else if (c == '\n')
+				lines++;
+		}
+		extra -= hdr_crs + bdy_crs;
+		if (out_cr)
+			extra += lines;
+	}
+
+	uint dummy_msg_len = 0;
+	char dummy_msg_buf[256];
+	static const char dummy_pfx[] = "[placeholder] ";
+	static const char dummy_subj[] = "Subject: [placeholder] (No Subject)";
+	static const char dummy_msg[] =
+		"Having a size of %s, this message is over the MaxSize limit.%s"
+		"Flag it and sync again (Sync mode Upgrade) to fetch its real contents.%s";
+	static const char dummy_flag[] =
+		"%s"
+		"The original message is flagged as important.%s";
+
+	if (vars->minimal) {
+		char sz[32];
+
+		if (vars->msg->size < 1024000)
+			sprintf( sz, "%dKiB", (int)(vars->msg->size >> 10) );
+		else
+			sprintf( sz, "%.1fMiB", vars->msg->size / 1048576. );
+		const char *nl = app_cr ? "\r\n" : "\n";
+		dummy_msg_len = (uint)sprintf( dummy_msg_buf, dummy_msg, sz, nl, nl );
+		if (vars->data.flags & F_FLAGGED) {
+			vars->data.flags &= ~F_FLAGGED;
+			dummy_msg_len += (uint)sprintf( dummy_msg_buf + dummy_msg_len, dummy_flag, nl, nl );
+		}
+		extra += dummy_msg_len;
+		extra += add_subj ? strlen(dummy_subj) + app_cr + 1 : strlen(dummy_pfx);
+	}
+
+	vars->data.len = in_len + extra;
+	if (vars->data.len > INT_MAX) {
+		free( in_buf );
+		return "is too big after conversion";
+	}
+	char *out_buf = vars->data.data = nfmalloc( vars->data.len );
+	idx = 0;
+	if (vars->srec) {
+		if (break2 < sbreak) {
+			copy_msg_bytes( &out_buf, in_buf, &idx, break2, in_cr, out_cr );
+			memcpy( out_buf, dummy_pfx, strlen(dummy_pfx) );
+			out_buf += strlen(dummy_pfx);
+		}
+		copy_msg_bytes( &out_buf, in_buf, &idx, sbreak, in_cr, out_cr );
+
+		memcpy( out_buf, "X-TUID: ", 8 );
+		out_buf += 8;
+		memcpy( out_buf, vars->srec->tuid, TUIDL );
+		out_buf += TUIDL;
+		if (app_cr)
+			*out_buf++ = '\r';
+		*out_buf++ = '\n';
+		idx = ebreak;
+
+		if (break2 != UINT_MAX && break2 >= sbreak) {
+			copy_msg_bytes( &out_buf, in_buf, &idx, break2, in_cr, out_cr );
+			if (!add_subj) {
+				memcpy( out_buf, dummy_pfx, strlen(dummy_pfx) );
+				out_buf += strlen(dummy_pfx);
+			} else {
+				memcpy( out_buf, dummy_subj, strlen(dummy_subj) );
+				out_buf += strlen(dummy_subj);
+				if (app_cr)
+					*out_buf++ = '\r';
+				*out_buf++ = '\n';
+			}
+		}
+	}
+	copy_msg_bytes( &out_buf, in_buf, &idx, in_len, in_cr, out_cr );
+
+	if (vars->minimal)
+		memcpy( out_buf, dummy_msg_buf, dummy_msg_len );
+
+	free( in_buf );
+	return NULL;
+}
diff --git a/src/sync_p.h b/src/sync_p.h
index ada2084..66f09e4 100644
--- a/src/sync_p.h
+++ b/src/sync_p.h
@@ -103,3 +103,14 @@ void assign_tuid( sync_vars_t *svars, sync_rec_t *srec );
 int match_tuids( sync_vars_t *svars, int t, message_t *msgs );
 
 sync_rec_t *upgrade_srec( sync_vars_t *svars, sync_rec_t *srec, int t );
+
+typedef struct copy_vars {
+	void (*cb)( int sts, uint uid, struct copy_vars *vars );
+	void *aux;
+	sync_rec_t *srec; /* also ->tuid */
+	message_t *msg;
+	msg_data_t data;
+	int minimal;
+} copy_vars_t;
+
+char *copy_msg_convert( int in_cr, int out_cr, copy_vars_t *vars );