diff --git a/store/account.go b/store/account.go
index 3ce57f0..e665776 100644
--- a/store/account.go
+++ b/store/account.go
@@ -1405,7 +1405,7 @@ func (a *Account) DeliverMessage(log mlog.Log, tx *bstore.Tx, m *Message, msgFil
 		}
 	}
 
-	// todo: perhaps we should match the recipients based on smtp submission and a matching message-id? we now miss the addresses in bcc's. for webmail, we could insert the recipients directly.
+	// todo: perhaps we should match the recipients based on smtp submission and a matching message-id? we now miss the addresses in bcc's if the mail client doesn't save a message that includes the bcc header in the sent mailbox.
 	if mb.Sent && part != nil && part.Envelope != nil {
 		e := part.Envelope
 		sent := e.Date
diff --git a/webapi/webapi.go b/webapi/webapi.go
index 16e1b82..1e5bda6 100644
--- a/webapi/webapi.go
+++ b/webapi/webapi.go
@@ -64,7 +64,8 @@ type Message struct {
 	To []NameAddress
 	CC []NameAddress
 	// For submissions, BCC addressees receive the message but are not added to the
-	// message headers. For incoming messages, this is typically empty.
+	// headers of the outgoing message. Only the message saved the to the Sent mailbox
+	// gets the Bcc header prepended. For incoming messages, this is typically empty.
 	BCC []NameAddress
 
 	// Optional Reply-To header, where the recipient is asked to send replies to.
diff --git a/webapisrv/server.go b/webapisrv/server.go
index 6f74c63..a5afc1b 100644
--- a/webapisrv/server.go
+++ b/webapisrv/server.go
@@ -615,7 +615,7 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 	from, fromPath := froms[0], fromPaths[0]
 	to, toPaths := xparseAddresses(m.To)
 	cc, ccPaths := xparseAddresses(m.CC)
-	_, bccPaths := xparseAddresses(m.BCC)
+	bcc, bccPaths := xparseAddresses(m.BCC)
 
 	recipients := append(append(toPaths, ccPaths...), bccPaths...)
 	addresses := append(append(m.To, m.CC...), m.BCC...)
@@ -714,6 +714,7 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 	}
 	xc.HeaderAddrs("To", to)
 	xc.HeaderAddrs("Cc", cc)
+	// We prepend Bcc headers to the message when adding to the Sent mailbox.
 	if m.Subject != "" {
 		xcheckcontrol(m.Subject)
 		xc.Subject(m.Subject)
@@ -1037,6 +1038,17 @@ func (s server) Send(ctx context.Context, req webapi.SendRequest) (resp webapi.S
 				modseq, err := acc.NextModSeq(tx)
 				xcheckf(err, "next modseq")
 
+				// If there were bcc headers, prepend those to the stored message only, before the
+				// DKIM signature. The DKIM-signature oversigns the bcc header, so this stored message
+				// won't validate with DKIM anymore, which is fine.
+				if len(bcc) > 0 {
+					var sb strings.Builder
+					xbcc := message.NewComposer(&sb, 100*1024, smtputf8)
+					xbcc.HeaderAddrs("Bcc", bcc)
+					xbcc.Flush()
+					msgPrefix = sb.String() + msgPrefix
+				}
+
 				sentm := store.Message{
 					CreateSeq:     modseq,
 					ModSeq:        modseq,
diff --git a/webapisrv/server_test.go b/webapisrv/server_test.go
index 5fb3886..1120caa 100644
--- a/webapisrv/server_test.go
+++ b/webapisrv/server_test.go
@@ -382,7 +382,6 @@ func TestServer(t *testing.T) {
 	msgRes, err := client.MessageGet(ctxbg, webapi.MessageGetRequest{MsgID: 1})
 	tcheckf(t, err, "remove suppressed address")
 	sentMsg := sendReq.Message
-	sentMsg.BCC = []webapi.NameAddress{} // todo: the Sent message should contain the BCC. for webmail too.
 	sentMsg.Date = msgRes.Message.Date
 	sentMsg.HTML += "\n"
 	tcompare(t, msgRes.Message, sentMsg)
diff --git a/webmail/api.go b/webmail/api.go
index 01a944d..88eedc3 100644
--- a/webmail/api.go
+++ b/webmail/api.go
@@ -286,7 +286,8 @@ func xrandom(ctx context.Context, n int) []byte {
 // Bcc message header.
 //
 // If a Sent mailbox is configured, messages are added to it after submitting
-// to the delivery queue.
+// to the delivery queue. If Bcc addresses were present, a header is prepended
+// to the message stored in the Sent mailbox.
 func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 	// Similar between ../smtpserver/server.go:/submit\( and ../webmail/api.go:/MessageSubmit\( and ../webapisrv/server.go:/Send\(
 
@@ -343,9 +344,11 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 		recipients = append(recipients, addr.Address)
 	}
 
+	var bccAddrs []message.NameAddress
 	for _, s := range m.Bcc {
 		addr, err := parseAddress(s)
 		xcheckuserf(ctx, err, "parsing Bcc address")
+		bccAddrs = append(bccAddrs, addr)
 		recipients = append(recipients, addr.Address)
 	}
 
@@ -446,6 +449,7 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 	}
 	xc.HeaderAddrs("To", toAddrs)
 	xc.HeaderAddrs("Cc", ccAddrs)
+	// We prepend Bcc headers to the message when adding to the Sent mailbox.
 	if m.Subject != "" {
 		xc.Subject(m.Subject)
 	}
@@ -750,6 +754,17 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 				xcheckf(ctx, err, "next modseq")
 			}
 
+			// If there were bcc headers, prepend those to the stored message only, before the
+			// DKIM signature. The DKIM-signature oversigns the bcc header, so this stored
+			// message won't validate with DKIM anymore, which is fine.
+			if len(bccAddrs) > 0 {
+				var sb strings.Builder
+				xbcc := message.NewComposer(&sb, 100*1024, smtputf8)
+				xbcc.HeaderAddrs("Bcc", bccAddrs)
+				xbcc.Flush()
+				msgPrefix = sb.String() + msgPrefix
+			}
+
 			sentm := store.Message{
 				CreateSeq:     modseq,
 				ModSeq:        modseq,
diff --git a/webmail/api.json b/webmail/api.json
index d6958bc..5b9a71d 100644
--- a/webmail/api.json
+++ b/webmail/api.json
@@ -101,7 +101,7 @@
 		},
 		{
 			"Name": "MessageSubmit",
-			"Docs": "MessageSubmit sends a message by submitting it the outgoing email queue. The\nmessage is sent to all addresses listed in the To, Cc and Bcc addresses, without\nBcc message header.\n\nIf a Sent mailbox is configured, messages are added to it after submitting\nto the delivery queue.",
+			"Docs": "MessageSubmit sends a message by submitting it the outgoing email queue. The\nmessage is sent to all addresses listed in the To, Cc and Bcc addresses, without\nBcc message header.\n\nIf a Sent mailbox is configured, messages are added to it after submitting\nto the delivery queue. If Bcc addresses were present, a header is prepended\nto the message stored in the Sent mailbox.",
 			"Params": [
 				{
 					"Name": "m",
diff --git a/webmail/api.ts b/webmail/api.ts
index 2b8c9a3..931d3c5 100644
--- a/webmail/api.ts
+++ b/webmail/api.ts
@@ -715,7 +715,8 @@ export class Client {
 	// Bcc message header.
 	// 
 	// If a Sent mailbox is configured, messages are added to it after submitting
-	// to the delivery queue.
+	// to the delivery queue. If Bcc addresses were present, a header is prepended
+	// to the message stored in the Sent mailbox.
 	async MessageSubmit(m: SubmitMessage): Promise<void> {
 		const fn: string = "MessageSubmit"
 		const paramTypes: string[][] = [["SubmitMessage"]]
diff --git a/webmail/api_test.go b/webmail/api_test.go
index 7919514..f1b551e 100644
--- a/webmail/api_test.go
+++ b/webmail/api_test.go
@@ -424,7 +424,7 @@ func TestAPI(t *testing.T) {
 	tcompare(t, len(l), 0)
 	tcompare(t, full, true)
 	l, full = api.CompleteRecipient(ctx, "cc2")
-	tcompare(t, l, []string{"mjl cc2 <mjl+cc2@mox.example>"})
+	tcompare(t, l, []string{"mjl cc2 <mjl+cc2@mox.example>", "mjl bcc2 <mjl+bcc2@mox.example>"})
 	tcompare(t, full, true)
 
 	// RecipientSecurity
diff --git a/webmail/msg.js b/webmail/msg.js
index 94c14d1..57dda5d 100644
--- a/webmail/msg.js
+++ b/webmail/msg.js
@@ -457,7 +457,8 @@ var api;
 		// Bcc message header.
 		// 
 		// If a Sent mailbox is configured, messages are added to it after submitting
-		// to the delivery queue.
+		// to the delivery queue. If Bcc addresses were present, a header is prepended
+		// to the message stored in the Sent mailbox.
 		async MessageSubmit(m) {
 			const fn = "MessageSubmit";
 			const paramTypes = [["SubmitMessage"]];
diff --git a/webmail/text.js b/webmail/text.js
index ab98a22..30cf66e 100644
--- a/webmail/text.js
+++ b/webmail/text.js
@@ -457,7 +457,8 @@ var api;
 		// Bcc message header.
 		// 
 		// If a Sent mailbox is configured, messages are added to it after submitting
-		// to the delivery queue.
+		// to the delivery queue. If Bcc addresses were present, a header is prepended
+		// to the message stored in the Sent mailbox.
 		async MessageSubmit(m) {
 			const fn = "MessageSubmit";
 			const paramTypes = [["SubmitMessage"]];
diff --git a/webmail/webmail.js b/webmail/webmail.js
index f34889e..ed96552 100644
--- a/webmail/webmail.js
+++ b/webmail/webmail.js
@@ -457,7 +457,8 @@ var api;
 		// Bcc message header.
 		// 
 		// If a Sent mailbox is configured, messages are added to it after submitting
-		// to the delivery queue.
+		// to the delivery queue. If Bcc addresses were present, a header is prepended
+		// to the message stored in the Sent mailbox.
 		async MessageSubmit(m) {
 			const fn = "MessageSubmit";
 			const paramTypes = [["SubmitMessage"]];