diff --git a/doc.go b/doc.go
index f895ca9..d7abf40 100644
--- a/doc.go
+++ b/doc.go
@@ -413,6 +413,8 @@ possibly making them potentially no longer readable by the previous version.
 	usage: mox verifydata data-dir
 	  -fix
 	    	fix fixable problems, such as moving away message files not referenced by their database
+	  -skip-size-check
+	    	skip the check for message size
 
 # mox config test
 
diff --git a/gentestdata.go b/gentestdata.go
index 16b657f..4dbb1ba 100644
--- a/gentestdata.go
+++ b/gentestdata.go
@@ -356,7 +356,7 @@ Accounts:
 			MailboxDestinedID: sent.ID,
 			Flags:             store.Flags{Seen: true, Junk: true},
 			Size:              int64(len(prefix1) + len(msg1)),
-			MsgPrefix:         []byte(prefix),
+			MsgPrefix:         []byte(prefix1),
 		}
 		mf1 := tempfile()
 		xcheckf(err, "creating temp file for delivery")
diff --git a/main.go b/main.go
index e4903a4..468844b 100644
--- a/main.go
+++ b/main.go
@@ -158,6 +158,7 @@ var commands = []struct {
 	{"gentestdata", cmdGentestdata},
 	{"ximport maildir", cmdXImportMaildir},
 	{"ximport mbox", cmdXImportMbox},
+	{"openaccounts", cmdOpenaccounts},
 }
 
 var cmds []cmd
@@ -2378,3 +2379,27 @@ func cmdMessageParse(c *cmd) {
 	err = enc.Encode(part)
 	xcheckf(err, "write")
 }
+
+func cmdOpenaccounts(c *cmd) {
+	c.unlisted = true
+	c.params = "datadir account ..."
+	c.help = `Open and close accounts, for triggering data upgrades, for tests.
+
+Opens database files directly, not going through a running mox instance.
+`
+
+	args := c.Parse()
+	if len(args) <= 1 {
+		c.Usage()
+	}
+
+	dataDir := filepath.Clean(args[0])
+	for _, accName := range args[1:] {
+		accDir := filepath.Join(dataDir, "accounts", accName)
+		log.Printf("opening account %s...", filepath.Join(accDir, accName))
+		a, err := store.OpenAccountDB(accDir, accName)
+		xcheckf(err, "open account %s", accName)
+		err = a.Close()
+		xcheckf(err, "close account %s", accName)
+	}
+}
diff --git a/store/account.go b/store/account.go
index 559c014..08cbe55 100644
--- a/store/account.go
+++ b/store/account.go
@@ -677,7 +677,6 @@ func OpenAccount(name string) (*Account, error) {
 	if err != nil {
 		return nil, err
 	}
-	acc.nused++
 	openAccounts.names[name] = acc
 	return acc, nil
 }
@@ -685,13 +684,20 @@ func OpenAccount(name string) (*Account, error) {
 // openAccount opens an existing account, or creates it if it is missing.
 func openAccount(name string) (a *Account, rerr error) {
 	dir := filepath.Join(mox.DataDirPath("accounts"), name)
-	dbpath := filepath.Join(dir, "index.db")
+	return OpenAccountDB(dir, name)
+}
+
+// OpenAccountDB opens an account database file and returns an initialized account
+// or error. Only exported for use by subcommands that verify the database file.
+// Almost all account opens must go through OpenAccount/OpenEmail/OpenEmailAuth.
+func OpenAccountDB(accountDir, accountName string) (a *Account, rerr error) {
+	dbpath := filepath.Join(accountDir, "index.db")
 
 	// Create account if it doesn't exist yet.
 	isNew := false
 	if _, err := os.Stat(dbpath); err != nil && os.IsNotExist(err) {
 		isNew = true
-		os.MkdirAll(dir, 0770)
+		os.MkdirAll(accountDir, 0770)
 	}
 
 	db, err := bstore.Open(context.TODO(), dbpath, &bstore.Options{Timeout: 5 * time.Second, Perm: 0660}, DBTypes...)
@@ -719,7 +725,7 @@ func openAccount(name string) (a *Account, rerr error) {
 			return bstore.QueryTx[Mailbox](tx).FilterEqual("HaveCounts", false).ForEach(func(mb Mailbox) error {
 				if !mentioned {
 					mentioned = true
-					xlog.Info("first calculation of mailbox counts for account", mlog.Field("account", name))
+					xlog.Info("first calculation of mailbox counts for account", mlog.Field("account", accountName))
 				}
 				mc, err := mb.CalculateCounts(tx)
 				if err != nil {
@@ -736,10 +742,11 @@ func openAccount(name string) (a *Account, rerr error) {
 	}
 
 	return &Account{
-		Name:   name,
-		Dir:    dir,
+		Name:   accountName,
+		Dir:    accountDir,
 		DBPath: dbpath,
 		DB:     db,
+		nused:  1,
 	}, nil
 }
 
diff --git a/test-upgrade.sh b/test-upgrade.sh
index a5b63b7..c82138f 100755
--- a/test-upgrade.sh
+++ b/test-upgrade.sh
@@ -22,6 +22,7 @@ cd testdata/upgrade
 # Check that we can upgrade what we currently generate.
 ../../mox gentestdata data
 ../../mox verifydata data
+../../mox openaccounts data test0 test1 test2
 rm -r data
 echo
 
@@ -38,8 +39,13 @@ for tag in $tagsrev; do
 	(CGO_ENABLED=0 GOBIN=$PWD/$tag go install github.com/mjl-/mox@$tag)
 	# Generate with historic release.
 	./$tag/mox gentestdata $tag/data
-	# Verify with current code.
-	../../mox verifydata $tag/data
+	# Verify with current code. v0.0.[45] had a message with wrong Size. We don't
+	# want to abort the upgrade check because of it.
+	if test $tag = v0.0.4 -o $tag = v0.0.5; then
+		../../mox verifydata -skip-size-check $tag/data
+	else
+		../../mox verifydata $tag/data
+	fi
 	echo
 	rm -r $tag/data
 done
@@ -65,12 +71,18 @@ for tag in $tags; do
 		fi
 
 		echo "Upgrade data to $tag."
-		time ./$tag/mox verifydata stepdata
+		if test $tag = v0.0.4 -o $tag = v0.0.5; then
+			time ./$tag/mox verifydata stepdata
+		else
+			time ./$tag/mox verifydata -skip-size-check stepdata
+			time ./$tag/mox openaccounts stepdata test0 test1 test2
+		fi
 		echo
 	fi
 done
 echo "Testing final upgrade to current."
-time ../../mox verifydata stepdata
+time ../../mox verifydata -skip-size-check stepdata
+time ../../mox openaccounts stepdata test0 test1 test2
 rm -r stepdata
 rm */mox
 cd ../..
diff --git a/verifydata.go b/verifydata.go
index b1357b9..3479646 100644
--- a/verifydata.go
+++ b/verifydata.go
@@ -46,6 +46,12 @@ possibly making them potentially no longer readable by the previous version.
 `
 	var fix bool
 	c.flag.BoolVar(&fix, "fix", false, "fix fixable problems, such as moving away message files not referenced by their database")
+
+	// To prevent aborting the upgrade test with v0.0.[45] that had a message with
+	// incorrect Size.
+	var skipSizeCheck bool
+	c.flag.BoolVar(&skipSizeCheck, "skip-size-check", false, "skip the check for message size")
+
 	args := c.Parse()
 	if len(args) != 1 {
 		c.Usage()
@@ -140,7 +146,7 @@ possibly making them potentially no longer readable by the previous version.
 	checkFile := func(dbpath, path string, prefixSize int, size int64) {
 		st, err := os.Stat(path)
 		checkf(err, path, "checking if file exists")
-		if err == nil && int64(prefixSize)+st.Size() != size {
+		if !skipSizeCheck && err == nil && int64(prefixSize)+st.Size() != size {
 			filesize := st.Size()
 			checkf(fmt.Errorf("%s: message size is %d, should be %d (length of MsgPrefix %d + file size %d), see \"mox fixmsgsize\"", path, size, int64(prefixSize)+st.Size(), prefixSize, filesize), dbpath, "checking message size")
 		}