diff --git a/src/engine/imap-engine/imap-engine-generic-account.vala b/src/engine/imap-engine/imap-engine-generic-account.vala
index c7a28c75..bddfe1f8 100644
--- a/src/engine/imap-engine/imap-engine-generic-account.vala
+++ b/src/engine/imap-engine/imap-engine-generic-account.vala
@@ -416,7 +416,7 @@ private abstract class Geary.ImapEngine.GenericAccount : Geary.Account {
         char type = (char) serialised.get_child_value(0).get_byte();
         if (type == 'i')
             return new ImapDB.EmailIdentifier.from_variant(serialised);
-        if (type == 's')
+        if (type == 'o')
             return new Outbox.EmailIdentifier.from_variant(serialised);
 
         throw new EngineError.BAD_PARAMETERS("Unknown serialised type: %c", type);
diff --git a/src/engine/outbox/outbox-email-identifier.vala b/src/engine/outbox/outbox-email-identifier.vala
index f3b93a62..cf12b751 100644
--- a/src/engine/outbox/outbox-email-identifier.vala
+++ b/src/engine/outbox/outbox-email-identifier.vala
@@ -45,7 +45,7 @@ private class Geary.Outbox.EmailIdentifier : Geary.EmailIdentifier {
         // Return a tuple to satisfy the API contract, add an 's' to
         // inform GenericAccount that it's an SMTP id.
         return new GLib.Variant.tuple(new Variant[] {
-                new GLib.Variant.byte('s'),
+                new GLib.Variant.byte('o'),
                 new GLib.Variant.int64(this.message_id),
                 new GLib.Variant.int64(this.ordering)
             });
diff --git a/test/engine/imap-engine/imap-engine-generic-account-test.vala b/test/engine/imap-engine/imap-engine-generic-account-test.vala
index 73f9793b..54829e15 100644
--- a/test/engine/imap-engine/imap-engine-generic-account-test.vala
+++ b/test/engine/imap-engine/imap-engine-generic-account-test.vala
@@ -88,6 +88,9 @@ public class Geary.ImapEngine.GenericAccountTest : TestCase {
         assert_non_null(
             test_article.to_email_identifier(new GLib.Variant("(yxx)", 'i', 1, 2))
         );
+        assert_non_null(
+            test_article.to_email_identifier(new GLib.Variant("(yxx)", 'o', 1, 2))
+        );
     }
 
 }