This (and other reasons) is why you don't use your ISP's email service.
Just wait until your ISP decides to change its name .. and its domain name.
Or when your ISP shuts down for 4 hrs to do an upgrade, and doesn't comes back! They were installing their own new email server, and the person they were leasing service from threw a fit, pushed The Big Red Button prematurely, and made everybody's email accounts disappear. 2 weeks later, they had their system back (I have no idea what OS they were running). I also had a tech friend who signed up for a cable ISP. When the installer showed up with the modem, he took one look at the server rack in his basement, the certifications on the wall, and suggested he NOT install the user program, but didn't say why.
“Cannot find REALITY.SYS...Universe Halted.” ~ God on the phone with Microsoft Customer Support