diff --git a/upload/upload.go b/upload/upload.go
index 395701c8..4413b41c 100644
--- a/upload/upload.go
+++ b/upload/upload.go
@@ -18,6 +18,7 @@ import (
 	awsconfig "github.com/aws/aws-sdk-go-v2/config"
 	"github.com/aws/aws-sdk-go-v2/credentials"
 	"github.com/aws/aws-sdk-go-v2/feature/s3/transfermanager"
+	"github.com/aws/aws-sdk-go-v2/feature/s3/transfermanager/types"
 	"github.com/aws/aws-sdk-go-v2/service/s3"
 	"github.com/neicnordic/crypt4gh/keys"
 	"github.com/spf13/cobra"
@@ -229,6 +230,9 @@ func uploadFiles(files, outFiles []string, targetDir string, config *helpers.Con
 			Bucket:          aws.String(config.AccessKey),
 			Key:             aws.String(path.Join(targetDir, outFiles[k])),
 			ContentEncoding: aws.String(config.Encoding),
+		}, func(o *transfermanager.Options) {
+			// Preserve RequestChecksumCalculationWhenRequired for S3-compatible inboxes.
+			o.ChecksumAlgorithm = types.ChecksumAlgorithm("")
 		})
 		// Print the progress bar. Second check is to filter out some junk from the output
 		if result != nil && result.VersionID != nil {
diff --git a/upload/upload_test.go b/upload/upload_test.go
index 28ccd714..7b10a2af 100644
--- a/upload/upload_test.go
+++ b/upload/upload_test.go
@@ -6,10 +6,12 @@ import (
 	"crypto/rsa"
 	"fmt"
 	"io"
+	"net/http"
 	"net/http/httptest"
 	"os"
 	"path/filepath"
 	"runtime"
+	"sync"
 	"testing"
 	"time"
 
@@ -36,6 +38,8 @@ type UploadTestSuite struct {
 	publicKeyFilePath  string
 	s3MockHTTPServer   *httptest.Server
 	s3Client           *s3.Client
+	objectPutHeaders   http.Header
+	objectPutHeadersMu sync.Mutex
 }
 
 var configFileFormat = `
@@ -70,12 +74,24 @@ func (s *UploadTestSuite) SetupTest() {
 	uploadCmd.Flag("access-token").Value.Set("")
 	os.Args = []string{"", "upload"}
 
+	s.objectPutHeadersMu.Lock()
+	s.objectPutHeaders = nil
+	s.objectPutHeadersMu.Unlock()
+
 	s.accessToken = s.generateDummyToken()
 	s.tempDir = s.T().TempDir()
 
 	backend := s3mem.New()
 	faker := gofakes3.New(backend)
-	s.s3MockHTTPServer = httptest.NewServer(faker.Server())
+	fakeS3Handler := faker.Server()
+	s.s3MockHTTPServer = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == http.MethodPut && r.URL.Path != "/dummy" {
+			s.objectPutHeadersMu.Lock()
+			s.objectPutHeaders = r.Header.Clone()
+			s.objectPutHeadersMu.Unlock()
+		}
+		fakeS3Handler.ServeHTTP(w, r)
+	}))
 
 	awsConfig, err := config.LoadDefaultConfig(context.TODO(),
 		config.WithCredentialsProvider(credentials.NewStaticCredentialsProvider("dummy", "dummy", s.accessToken)),
@@ -275,6 +291,23 @@ func (s *UploadTestSuite) TestUploadTargetDir() {
 	}
 	assert.Equal(s.T(), aws.ToString(result.Contents[0].Key), fmt.Sprintf("%s/%s", filepath.ToSlash(targetPath), filepath.Base(s.uploadTestFilePath)))
 }
+
+func (s *UploadTestSuite) TestUploadSinglePartDoesNotSendFlexibleChecksumHeaders() {
+	os.Args = []string{"", "upload", s.uploadTestFilePath}
+	uploadCmd.Flag("force-unencrypted").Value.Set("true")
+	assert.NoError(s.T(), uploadCmd.Execute())
+
+	s.objectPutHeadersMu.Lock()
+	headers := s.objectPutHeaders.Clone()
+	s.objectPutHeadersMu.Unlock()
+
+	if assert.NotNil(s.T(), headers, "expected object PUT request headers to be captured") {
+		assert.Empty(s.T(), headers.Values("X-Amz-Sdk-Checksum-Algorithm"))
+		assert.Empty(s.T(), headers.Values("X-Amz-Checksum-Crc32"))
+		assert.Empty(s.T(), headers.Values("X-Amz-Trailer"))
+	}
+}
+
 func (s *UploadTestSuite) TestUploadWithEncryption() {
 	rescuedStdout := os.Stdout
 	stdoutReader, stdoutWriter, _ := os.Pipe()