Strictly speaking, no, you should not experience any down time.  However, your IT professional would be the best person to answer that.

As a general rule, we recommend setting up all mailboxes in advance of the cutover.  Once the cutover takes place (by changing the MX record assoicated with your domain), e-mail will start flowing to our servers as opposed to your old servers.

If you are moving from POP3 to our Exchange services, the migration is actually really easy.  If you are moving from your own Exchange server to us the process is a little more complicated, but still very straight forward for an IT person.