diff --git a/go.mod b/go.mod
index 2f63a62..47852cb 100644
--- a/go.mod
+++ b/go.mod
@@ -7,6 +7,7 @@ require (
 	github.com/chris-ramon/douceur v0.2.0
 	github.com/emersion/go-imap v1.0.3
 	github.com/emersion/go-imap-move v0.0.0-20190710073258-6e5a51a5b342
+	github.com/emersion/go-imap-specialuse v0.0.0-20161227184202-ba031ced6a62
 	github.com/emersion/go-message v0.11.1
 	github.com/emersion/go-sasl v0.0.0-20191210011802-430746ea8b9b
 	github.com/emersion/go-smtp v0.12.1
diff --git a/go.sum b/go.sum
index 3fb32fe..a056692 100644
--- a/go.sum
+++ b/go.sum
@@ -14,6 +14,8 @@ github.com/emersion/go-imap v1.0.3 h1:5eEee8/DTSIPfliiWqwfvjPGkU8bBtvOy/Wx+eeXzO
 github.com/emersion/go-imap v1.0.3/go.mod h1:yKASt+C3ZiDAiCSssxg9caIckWF/JG7ZQTO7GAmvicU=
 github.com/emersion/go-imap-move v0.0.0-20190710073258-6e5a51a5b342 h1:5p1t3e1PomYgLWwEwhwEU5kVBwcyAcVrOpexv8AeZx0=
 github.com/emersion/go-imap-move v0.0.0-20190710073258-6e5a51a5b342/go.mod h1:QuMaZcKFDVI0yCrnAbPLfbwllz1wtOrZH8/vZ5yzp4w=
+github.com/emersion/go-imap-specialuse v0.0.0-20161227184202-ba031ced6a62 h1:4ZAfwfc8aDlj26kkEap1UDSwwDnJp9Ie8Uj1MSXAkPk=
+github.com/emersion/go-imap-specialuse v0.0.0-20161227184202-ba031ced6a62/go.mod h1:/nybxhI8kXom8Tw6BrHMl42usALvka6meORflnnYwe4=
 github.com/emersion/go-message v0.11.1 h1:0C/S4JIXDTSfXB1vpqdimAYyK4+79fgEAMQ0dSL+Kac=
 github.com/emersion/go-message v0.11.1/go.mod h1:C4jnca5HOTo4bGN9YdqNQM9sITuT3Y0K6bSUw9RklvY=
 github.com/emersion/go-sasl v0.0.0-20190817083125-240c8404624e h1:ba7YsgX5OV8FjGi5ZWml8Jng6oBrJAb3ahqWMJ5Ce8Q=
diff --git a/plugins/base/imap.go b/plugins/base/imap.go
index 09ae305..0c39ca6 100755
--- a/plugins/base/imap.go
+++ b/plugins/base/imap.go
@@ -35,6 +35,30 @@ func listMailboxes(conn *imapclient.Client) ([]*imap.MailboxInfo, error) {
 	return mailboxes, nil
 }
 
+func getMailboxByAttribute(conn *imapclient.Client, attr string) (*imap.MailboxInfo, error) {
+	ch := make(chan *imap.MailboxInfo, 10)
+	done := make(chan error, 1)
+	go func() {
+		done <- conn.List("", "%", ch)
+	}()
+
+	var mailbox *imap.MailboxInfo
+	for mbox := range ch {
+		for _, a := range mbox.Attributes {
+			if attr == a {
+				mailbox = mbox
+				break
+			}
+		}
+	}
+
+	if err := <-done; err != nil {
+		return nil, fmt.Errorf("failed to get mailbox with attribute %q: %v", attr, err)
+	}
+
+	return mailbox, nil
+}
+
 func ensureMailboxSelected(conn *imapclient.Client, mboxName string) error {
 	mbox := conn.Mailbox()
 	if mbox == nil || mbox.Name != mboxName {
@@ -339,3 +363,15 @@ func getMessagePart(conn *imapclient.Client, mboxName string, uid uint32, partPa
 
 	return &IMAPMessage{msg}, part, nil
 }
+
+func markMessageAnswered(conn *imapclient.Client, mboxName string, uid uint32) error {
+	if err := ensureMailboxSelected(conn, mboxName); err != nil {
+		return err
+	}
+
+	seqSet := new(imap.SeqSet)
+	seqSet.AddNum(uid)
+	item := imap.FormatFlagsOp(imap.AddFlags, true)
+	flags := []interface{}{imap.AnsweredFlag}
+	return conn.UidStore(seqSet, item, flags, nil)
+}
diff --git a/plugins/base/routes.go b/plugins/base/routes.go
index ad93c6f..961f25a 100644
--- a/plugins/base/routes.go
+++ b/plugins/base/routes.go
@@ -1,6 +1,7 @@
 package koushinbase
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"mime"
@@ -8,10 +9,12 @@ import (
 	"net/url"
 	"strconv"
 	"strings"
+	"time"
 
 	"git.sr.ht/~emersion/koushin"
 	"github.com/emersion/go-imap"
 	imapmove "github.com/emersion/go-imap-move"
+	imapspecialuse "github.com/emersion/go-imap-specialuse"
 	imapclient "github.com/emersion/go-imap/client"
 	"github.com/emersion/go-message"
 	"github.com/emersion/go-smtp"
@@ -289,12 +292,19 @@ func handleCompose(ctx *koushin.Context) error {
 	msg.Text = ctx.QueryParam("body")
 	msg.InReplyTo = ctx.QueryParam("in-reply-to")
 
-	if ctx.Request().Method == http.MethodGet && ctx.Param("uid") != "" {
+	var inReplyToMboxName string
+	var inReplyToUid uint32
+	if ctx.Param("uid") != "" {
 		// This is a reply
-		mboxName, uid, err := parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
+		var err error
+		inReplyToMboxName, inReplyToUid, err = parseMboxAndUid(ctx.Param("mbox"), ctx.Param("uid"))
 		if err != nil {
 			return echo.NewHTTPError(http.StatusBadRequest, err)
 		}
+	}
+
+	if ctx.Request().Method == http.MethodGet && inReplyToUid != 0 {
+		// Populate fields from original message
 		partPath, err := parsePartPath(ctx.QueryParam("part"))
 		if err != nil {
 			return echo.NewHTTPError(http.StatusBadRequest, err)
@@ -304,7 +314,7 @@ func handleCompose(ctx *koushin.Context) error {
 		var part *message.Entity
 		err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
 			var err error
-			inReplyTo, part, err = getMessagePart(c, mboxName, uid, partPath)
+			inReplyTo, part, err = getMessagePart(c, inReplyToMboxName, inReplyToUid, partPath)
 			return err
 		})
 		if err != nil {
@@ -364,7 +374,39 @@ func handleCompose(ctx *koushin.Context) error {
 			if _, ok := err.(koushin.AuthError); ok {
 				return echo.NewHTTPError(http.StatusForbidden, err)
 			}
-			return err
+			return fmt.Errorf("failed to send message: %v", err)
+		}
+
+		if inReplyToUid != 0 {
+			err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
+				return markMessageAnswered(c, inReplyToMboxName, inReplyToUid)
+			})
+			if err != nil {
+				return fmt.Errorf("failed to mark original message as answered: %v", err)
+			}
+		}
+
+		err = ctx.Session.DoIMAP(func(c *imapclient.Client) error {
+			mbox, err := getMailboxByAttribute(c, imapspecialuse.Sent)
+			if err != nil {
+				return err
+			}
+			if mbox == nil {
+				return nil
+			}
+
+			// IMAP needs to know in advance the final size of the message, so
+			// there's no way around storing it in a buffer here.
+			var buf bytes.Buffer
+			if err := msg.WriteTo(&buf); err != nil {
+				return err
+			}
+
+			flags := []string{imap.SeenFlag}
+			return c.Append(mbox.Name, flags, time.Now(), &buf)
+		})
+		if err != nil {
+			return fmt.Errorf("failed to save message to Sent mailbox: %v", err)
 		}
 
 		// TODO: append to IMAP Sent mailbox