When Office 365 launched the mobile server address was m.outlook.com. This was a great address: easy to remember, pretty short, etc.
A couple months ago my phone stopped syncing my mail. I went to the Office 365 forums and found that for some reason m.outlook.com was now broken and everybody had to switch to an address found by going into Options -> See All Options -> Settings for POP, IMAP, and SMTP access... Mine was pod51009.outlook.com.
Then last night it went down again. I checked online and sure enough the address has now changed again. This time you have to use the hostname found in Help->About. Mine is ch1prd0402.outlook.com.
This is completely unacceptable as a paying customer. If you know that you are messing around with DNS or something that will change what server address your customers use, you should send out a heads up email and get people to switch over early. At the very least just a heads up email that it's broken and here is how to fix it. Letting the community find out how to fix it after Microsoft breaks it is getting old fast.
Update - January 25, 2012
Now that I'm using the actual hostname it can change at anytime. I figured this probably wouldn't change that often but I was wrong. It changed for me after 2 weeks. This is getting old pretty fast. Microsoft also seems to be either completely ignoring the issue or saying that it's affecting a small number of customers. I fail to see how every person who uses Android is a small number considering they now have the largest market share in smartphones.