diff --git a/imapserver/append_test.go b/imapserver/append_test.go
index a81071d..45a883f 100644
--- a/imapserver/append_test.go
+++ b/imapserver/append_test.go
@@ -55,11 +55,11 @@ func TestAppend(t *testing.T) {
 	tc3.transactf("ok", "noop")
 	tc3.xuntagged() // Inbox is not selected, nothing to report.
 
-	tc2.transactf("ok", "append inbox (\\Seen) \" 1-Jan-2022 10:10:00 +0100\" UTF8 ({47+}\r\ncontent-type: just completely invalid;;\r\n\r\ntest)")
+	tc2.transactf("ok", "append inbox (\\Seen) \" 1-Jan-2022 10:10:00 +0100\" UTF8 (~{47+}\r\ncontent-type: just completely invalid;;\r\n\r\ntest)")
 	tc2.xuntagged(imapclient.UntaggedExists(2))
 	tc2.xcodeArg(imapclient.CodeAppendUID{UIDValidity: 1, UIDs: xparseUIDRange("2")})
 
-	tc2.transactf("ok", "append inbox (\\Seen) \" 1-Jan-2022 10:10:00 +0100\" UTF8 ({31+}\r\ncontent-type: text/plain;\n\ntest)")
+	tc2.transactf("ok", "append inbox (\\Seen) \" 1-Jan-2022 10:10:00 +0100\" UTF8 (~{31+}\r\ncontent-type: text/plain;\n\ntest)")
 	tc2.xuntagged(imapclient.UntaggedExists(3))
 	tc2.xcodeArg(imapclient.CodeAppendUID{UIDValidity: 1, UIDs: xparseUIDRange("3")})
 
@@ -81,7 +81,7 @@ func TestAppend(t *testing.T) {
 
 	// Multiappend with two messages.
 	tc.transactf("ok", "noop") // Flush pending untagged responses.
-	tc.transactf("ok", "append inbox {6+}\r\ntest\r\n {6+}\r\ntost\r\n")
+	tc.transactf("ok", "append inbox {6+}\r\ntest\r\n ~{6+}\r\ntost\r\n")
 	tc.xuntagged(imapclient.UntaggedExists(5))
 	tc.xcodeArg(imapclient.CodeAppendUID{UIDValidity: 1, UIDs: xparseUIDRange("4:5")})
 
diff --git a/imapserver/server.go b/imapserver/server.go
index 95e8119..dd8d79b 100644
--- a/imapserver/server.go
+++ b/imapserver/server.go
@@ -3303,7 +3303,7 @@ func flaglist(fl store.Flags, keywords []string) listspace {
 //
 // State: Authenticated and selected.
 func (c *conn) cmdAppend(tag, cmd string, p *parser) {
-	// Command: ../rfc/9051:3406 ../rfc/6855:204 ../rfc/3501:2527 ../rfc/3502:95
+	// Command: ../rfc/9051:3406 ../rfc/6855:204 ../rfc/4466:427 ../rfc/3501:2527 ../rfc/3502:95
 	// Examples: ../rfc/9051:3482 ../rfc/3501:2589 ../rfc/3502:175
 
 	// A message that we've (partially) read from the client, and will be delivering to
@@ -3385,7 +3385,12 @@ func (c *conn) cmdAppend(tag, cmd string, p *parser) {
 		// todo: this is only relevant if we also support the CATENATE extension?
 		// ../rfc/6855:204
 		utf8 := p.take("UTF8 (")
-		size, synclit := p.xliteralSize(utf8, false)
+		if utf8 {
+			p.xtake("~")
+		}
+		// Always allow literal8, for binary extension. ../rfc/4466:486
+		// For utf8, we already consumed the required ~ above.
+		size, synclit := p.xliteralSize(!utf8, false)
 
 		if !quotaUnlimited && !overQuota {
 			quotaAvail -= size