Strange problem with scanner

Hi,

I have a mail server to which several Kyocera multifunction
scanner/printers are connected. When a document is scanned, these
devices can mail the scanned image through a mail account at the
server. They each include a mail client. The problem is that they all
complain about being unable to find the mail server. This happens at
totally unexpected times and then, they suddenly start working again,
as they normally have. The failure does not have a pattern: it may
happen once a month and twice in a single week or day.

Does any of you have an idea of what this problem might be, or have
run across a similar situation?

Thanks
Gerardo