Earlier quoted context omitted.
Fastmail.com (not affiliated)
Fastmail had interruptions few times and some lasted few hours. There is no such thing about 100% uptime of any service.
We use FastMail for a business that is sensitive to email delivery time — in other words, if for some reason we receive emails with a delay, this could be a problem. Over the past 5 years or so, we've had only one incident when emails weren't delivered on time.