It is always possible that your mail server is temporarily unavailable: maintenance, a failing internet connection or a broken server are a few of the possible causes.
To avoid losing mail, it is wise to use a backup mail server.
Especially if the mail server is unavailable for an extended period, the mail will eventually be sent back to the sender. And it is unprofessional if your customer notices there is a problem with your mail server.
Therefore, make use of the backup mail server facilities of bHosted.nl.