I host my own mail. When it’s down, the mail just gets delivered after I get online again. Almost all mail servers are configured to retry over a period of several days before giving up.
Once my health insurer sent me mail by post to tell me that my mail server was down. That was kinda funny.
The longest outage I’ve had in a decade is when my primary SSD died a 2 months ago and I had to reinstall using config backups. It was down for around a day.
I’ve thrown a UPS on it and flown overseas for a week or two. It’s basically just email for me and the kids.
I’ve had longer outages on hosted services, TBH.