Update docs/configuring-playbook-email2matrix.md: add the common section "Troubleshooting"

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-email2matrix.md

@@ -124,3 +124,15 @@ ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start
   `just install-service email2matrix` is useful for maintaining your setup quickly when its components remain unchanged. If you adjust your `vars.yml` to remove other components, you'd need to run `just setup-all`, or these components will still remain installed. Note `just setup-all` runs the `ensure-matrix-users-created` tag too.
 
 - After installation, you may wish to send a test email to the email address assigned to `mailbox1` (default: `mailbox1@matrix.example.com`) to make sure that Email2Matrix works as expected.
+
+## Troubleshooting
+
+As with all other services, you can find the logs in [systemd-journald](https://www.freedesktop.org/software/systemd/man/systemd-journald.service.html) by logging in to the server with SSH and running `journalctl -fu matrix-email2matrix`.
+
+### Increase logging verbosity
+
+If you want to increase the verbosity, add the following configuration to your `vars.yml` file and re-run the playbook:
+
+```yaml
+matrix_email2matrix_misc_debug: true
+```