Go with gmail man, it's setup to work just like a desktop email client so you can send and receive from multiple email addresses within the one interface. It also can be setup to automatically respond using the email address the original email was sent to. As well, they have recently added the ability to run multiple gmail addresses through the single account.
For example, my gmail accuount is
[email protected], but I also receive email for 3 other gmail addresses within that account. Additionally, I receive emails sent to about 15 other email addresses I have across about 5 domains. Each one is setup so that when I click "Reply" to an email, gmail automatically responds using the email address that the sender sent their email to.
It's a fantastic system and it's all web-based and of course gmail has virtually unlimited space... no more of that "oops my computer crashed and because I use outlook, I lost EVERYTHING".
Seriously, it's fantastic especially with how they keep adding to gmail with gmail labs, themes and google documents... she takes a little time to get used to but once your going, it makes life so much easier!