These customers I just created email accounts to forward from their server as needed and then tied their personal email accounts to the one location.
I have 10 customers using that. They were skeptics at first, but after they used it for a while they really like it.
Another idea for those who are using google apps for their customers
I usually create a sub domain off the server called mail.somedomain.com and then redirect to the google app url. So they only have to remember to put mail. in front of their domain.
Google apps allows desktop icons too, which I send them links to download. But if they are gone and way from their computer, they just got to remember the mail . thing and they are into their mail.
Kim