Here's a question: before I moved the client to this new host, she had amassed a mailbox well over the advised 2GB limit. Not the biggest I've seen, but at least 2.8GB. I didn't notice this until I'd backed up her data and then uploaded it to the new servers. I then worked with her to get it down to around 1.5GB (mostly by deleting thousands of sent items).

Could the contents of her mailbox cause the problem? I'm trying to figure out why her account is causing problems when her employees, who have FAR less data, don't seem to have any issues with the same service.

I've asked her to try using her laptop for the time being, and to try taking it somewhere with WiFi access. She's going to get back to me with the results.