diff --git a/message_import.go b/message_import.go
index 592ece2..884234c 100644
--- a/message_import.go
+++ b/message_import.go
@@ -19,8 +19,8 @@ const (
 	// maxImportCount is the maximum number of messages that can be imported in a single request.
 	maxImportCount = 10
 
-	// maxImportSize is the maximum total request size permitted for a single import request.
-	maxImportSize = 30 * 1024 * 1024
+	// MaxImportSize is the maximum total request size permitted for a single import request.
+	MaxImportSize = 30 * 1024 * 1024
 )
 
 var ErrImportEncrypt = errors.New("failed to encrypt message")
@@ -31,23 +31,28 @@ type ImportResStream stream.Stream[ImportRes] // gomock does not support generic
 func (c *Client) ImportMessages(ctx context.Context, addrKR *crypto.KeyRing, workers, buffer int, req ...ImportReq) (ImportResStream, error) {
 	// Encrypt each message.
 	for idx := range req {
-		enc, err := EncryptRFC822(addrKR, req[idx].Message)
+
+		// Encryption might mangle the original message bufer, so use a copy.
+		msgCopy := make([]byte, len(req[idx].Message))
+		copy(msgCopy, req[idx].Message)
+
+		enc, err := EncryptRFC822(addrKR, msgCopy)
 		if err != nil {
 			return nil, fmt.Errorf("%w %v: %v", ErrImportEncrypt, idx, err)
 		}
 
-		req[idx].Message = enc
+		req[idx].encryptedMessage = enc
 	}
 
 	// If any of the messages exceed the maximum import size, return an error.
-	if xslices.Any(req, func(req ImportReq) bool { return len(req.Message) > maxImportSize }) {
+	if xslices.Any(req, func(req ImportReq) bool { return len(req.encryptedMessage) > MaxImportSize }) {
 		return nil, ErrImportSizeExceeded
 	}
 
 	return stream.Flatten(parallel.MapStream(
 		ctx,
-		stream.FromIterator(iterator.Slice(ChunkSized(req, maxImportCount, maxImportSize, func(req ImportReq) int {
-			return len(req.Message)
+		stream.FromIterator(iterator.Slice(ChunkSized(req, maxImportCount, MaxImportSize, func(req ImportReq) int {
+			return len(req.encryptedMessage)
 		}))),
 		workers,
 		buffer,
diff --git a/message_import_types.go b/message_import_types.go
index 667898b..7f62f67 100644
--- a/message_import_types.go
+++ b/message_import_types.go
@@ -8,8 +8,13 @@ import (
 )
 
 type ImportReq struct {
-	Metadata ImportMetadata
-	Message  []byte
+	Metadata         ImportMetadata
+	Message          []byte
+	encryptedMessage []byte
+}
+
+func (r ImportReq) GetEncryptedMessageLength() int {
+	return len(r.encryptedMessage)
 }
 
 type namedImportReq struct {
@@ -42,7 +47,7 @@ func buildImportReqFields(req []namedImportReq) ([]*resty.MultipartField, error)
 			Param:       req.Name,
 			FileName:    req.Name + ".eml",
 			ContentType: string(rfc822.MessageRFC822),
-			Stream:      resty.NewByteMultipartStream(append(req.Message, "\r\n"...)),
+			Stream:      resty.NewByteMultipartStream(append(req.encryptedMessage, "\r\n"...)),
 		})
 	}
 
diff --git a/server/helper_test.go b/server/helper_test.go
index b17d610..d7575f0 100644
--- a/server/helper_test.go
+++ b/server/helper_test.go
@@ -2,6 +2,7 @@ package server
 
 import (
 	"fmt"
+	"strings"
 
 	"github.com/google/uuid"
 )
@@ -13,3 +14,8 @@ func newMessageLiteral(from, to string) []byte {
 func newMessageLiteralWithSubject(from, to, subject string) []byte {
 	return []byte(fmt.Sprintf("From: %v\r\nReceiver: %v\r\nSubject: %v\r\n\r\nHello World!", from, to, subject))
 }
+
+func newMessageLiteralWithSubjectAndSize(from, to, subject string, paddingSize int) []byte {
+	padding := strings.Repeat("A", paddingSize)
+	return []byte(fmt.Sprintf("From: %v\r\nReceiver: %v\r\nSubject: %v\r\n\r\nHello World!Padding:%s", from, to, subject, padding))
+}
diff --git a/server/server_test.go b/server/server_test.go
index 3195894..628e217 100644
--- a/server/server_test.go
+++ b/server/server_test.go
@@ -2700,3 +2700,72 @@ func getFullMessages(ctx context.Context,
 		},
 	)
 }
+
+func TestServer_Import_MessageSizeExceeded_NoDoubleEncryption(t *testing.T) {
+	withServer(t, func(ctx context.Context, s *Server, m *proton.Manager) {
+		withUser(ctx, t, s, m, "user", "pass", func(c *proton.Client) {
+			ctx, cancel := context.WithCancel(ctx)
+			defer cancel()
+
+			user, err := c.GetUser(ctx)
+			require.NoError(t, err)
+
+			addr, err := c.GetAddresses(ctx)
+			require.NoError(t, err)
+
+			salt, err := c.GetSalts(ctx)
+			require.NoError(t, err)
+
+			pass, err := salt.SaltForKey([]byte("pass"), user.Keys.Primary().ID)
+			require.NoError(t, err)
+
+			_, addrKRs, err := proton.Unlock(user, addr, pass, async.NoopPanicHandler{})
+			require.NoError(t, err)
+
+			reqs := []proton.ImportReq{
+				{
+					Metadata: proton.ImportMetadata{
+						AddressID: addr[0].ID,
+						LabelIDs:  []string{},
+						Flags:     proton.MessageFlagReceived,
+						Unread:    true,
+					},
+					Message: newMessageLiteralWithSubjectAndSize("test1@example.com", "test2@example.com", uuid.NewString(), 40),
+				},
+				{
+					Metadata: proton.ImportMetadata{
+						AddressID: addr[0].ID,
+						LabelIDs:  []string{},
+						Flags:     proton.MessageFlagReceived,
+						Unread:    true,
+					},
+					Message: newMessageLiteralWithSubjectAndSize("test1@example.com", "test2@example.com", uuid.NewString(), proton.MaxImportSize),
+				},
+				{
+					Metadata: proton.ImportMetadata{
+						AddressID: addr[0].ID,
+						LabelIDs:  []string{},
+						Flags:     proton.MessageFlagReceived,
+						Unread:    true,
+					},
+					Message: newMessageLiteralWithSubjectAndSize("test1@example.com", "test2@example.com", uuid.NewString(), 20),
+				},
+			}
+
+			reqsCopy := make([]proton.ImportReq, len(reqs))
+			for idx := range len(reqs) {
+				messageCopy := make([]byte, len(reqs[idx].Message))
+				copy(messageCopy, reqs[idx].Message)
+				reqsCopy[idx].Message = messageCopy
+			}
+
+			_, err = c.ImportMessages(ctx, addrKRs[addr[0].ID], runtime.NumCPU(), runtime.NumCPU(), reqs...)
+			require.ErrorIs(t, err, proton.ErrImportSizeExceeded)
+
+			for idx := range len(reqs) {
+				require.Equal(t, reqsCopy[idx].Message, reqs[idx].Message)
+				require.True(t, reqs[idx].GetEncryptedMessageLength() > 0)
+			}
+		})
+	})
+}