All MTAs have retries baked in, so running a self-hosted email server that receives mail is actually one of the most forgiving services in this respect. If your server is offline, the sending server will retry several times over 24-48 hours before it gives up. Even the big cloud email providers will do this.

That said, there are other aspects of running an email server that require some extra rigour, but they’re more on the sending side (making sure emails you send to other people actually land in their inbox). Doable, but one of the more challenging things to self-host.