I had a serious fault two years ago, when several forest fires knocked out electric power for the two lines feeding the datacenter. Electricity failed three or four times in short periods, and for some reason this wrecked havoc on the UPS-generator combo, and power went out. We were down for six hours and mad at the co-location company.

Other than that, thirty minutes downtime 14 months ago when the public-facing redundant routers mis-detected a failover, failed STONITH and entered split-brain status.

I wouldn't commit to better than 99.9% with our infrastructure (single datacenter with remote data backups) but we exceed the metric most years (99.99% in all but one of the last ten years).

I've been running my own mail and other services since 2005. We had one severe outage in 2008, caused by a bug in reiserfs. In 2013 I migrated to a different server, with minimal downtime but some work involved. Last year, one of the disks in the raid1 was giving smart errors, but it turned out it was hot swappable.

I can't recall the last unplanned outage since the 2008 one.

No outages except when I've migrated postfix/dovecot from old to new servers, and misconfigured. Otherwise uptime has been equal to server provider uptime (Rackspace, then AWS).