The Mail Source Agent or Mail Target Agent may report it's status as "pause pending", and cannot be stopped or started (the options are grayed out).
To resolve this issue, uninstall and re-install the agent's service:
Right-click your source-target server pair. Move the mouse over the agent's menu entry (either source or target agent), and click "uninstall service". Once this has completed, perform the same steps again, only select "install service". The agent should now show as being stopped, and can be started.
If this is not successful, reinstall the agent completely. See KB 25169 for details.