I will be taking the client mail server down just after midnight to image the machine so that if it is necessary to restore it will have all the recent updates in the image. This will take approximately 20 minutes.

During this time, web mail will be unavailable, it will not be possible to access mail via IMAP or POP3 mail clients, and you will not be able to send mail.

You will be able to read mail via shell mailers such as pine, elm, mutt, mush, and mailx during this time frame.  Outgoing mail sent via shell mailers should queue and be sent when the client mail server is returned to service.