If our email is down but our customers' email isn't down, then they think we're ignoring their "urgent" messages, and that's bad.
But if everyone's email is down, they won't be able to send those messages, so they won't think we received them to ignore them.
(Then again, email is kind of uniquely bad here, because of the way SMTP works as a store-and-forward protocol. In most protocols, if my server went down, your client wouldn't be able to connect to it, so it'd be pretty clear something is wrong. With SMTP, your client can just "put a message on the Internet" for my server to receive, and won't know for 48 hours or more that my server isn't there any more.)
I meant total downtime for each user. So if I lose email for an hour, and my worker loses email for an hour, it is better if those are the same hour, otherwise there are two hours when we can't email each other.
But if everyone's email is down, they won't be able to send those messages, so they won't think we received them to ignore them.
(Then again, email is kind of uniquely bad here, because of the way SMTP works as a store-and-forward protocol. In most protocols, if my server went down, your client wouldn't be able to connect to it, so it'd be pretty clear something is wrong. With SMTP, your client can just "put a message on the Internet" for my server to receive, and won't know for 48 hours or more that my server isn't there any more.)