If you open up the code in the plugin editor, on line 305, change the input type from "text" to "password":
<td><input name="smtp_pass" type="password" id="smtp_pass" value="<?php print(get_option('smtp_pass')); ?>" size="40" class="code" /></td>
That will make the password appear as discs instead of the actual password. Keep in mind that until the plugin author does this, you'll have to re-add the password type to the plugin every time it's upgraded.
Another thing worth mentioning is that anyone who has access to your plugins directory would be able to change it back. Your best bet would be to make sure no one but yourself can access sensitive information like that. It would also be a good idea to use an email address that's dedicated to emails from your website and not connected to anything else.