Is there an option to suspend mailflow?

For example, during a maintenance window of email servers, if we wanted to hold email for an hour. How would we do that?

You should not have to do anything. By default, most SMTP server will try sending messages if your machine is down. Although the actual interval for retrying is different for every server and something that you cannot control, most email servers will retry sending their emails for about at least few hours. Therefore, you don't have to do anything if your Xeams is down temporarily.

Another alternative is to use clustering in Xeams (http://xeams.com/clustering.htm) .This way if one server is down, other can receive emails from the Internet.