<i></i>* Mail you want to receive:
Your server can be down for a few days (if I remember correctly, 5 days, according to postfix defaults) and still receive all emails when it goes up again. More downtime and the senders will receive permanent error bounces.<p>If you fear longer server downtimes, you also have the option of secondary MX servers.<p><i></i>* Mail you send:
Just display the mail log (&#x2F;var&#x2F;log&#x2F;mail.info) and see if you get 250 OK responses when you submit messages. What happens next is not your responsibility anymore (except for spam filter rules of course, you need spf, dkim...)