I have been self-hosting my email for a few years now. (I use Zimbra BTW) Whenever the server goes down for whatever reason, the sending SMTP will retry. Each server has their own configuration, and can be changed by the adminsitrator, but it's usually a few days. As an example, on their default configurations, Exim will retry every 24h, and give up after a week. IIS will retry every 2 hours and give up after 2 days.