modem disconnects when checking email

A friend of mine called tonight with an unusual problem. He dials up to the
internet, and each time he can connect. However, when he goes to his ISP's
homepage and hits "read mail," the modem disconnects and starts redialing.

I know I've left out a lot of info, but is that general description enough
to allow someone to suggest a possible cause/solution?

Thank you,

jm