I spent a few days setting everything up and testing it. I flipped the switch and...silence. No incoming email for a few hours in the middle of a Friday night. CEO calls me at 2am asking why our prospective Hong Kong client's emails are bouncing back. Ooops...flip the DNS back to the old email server.
Call tech support. It's an Indian call center reading the tech support web site. After 20 minutes they escalate.
Wait 24 hours. "We're looking into it." Wait another 24 hours. "We've found the problem and are fixing it." Wait 48 hours.
"There is weird edge case bug with the code that manages the online signups. This affects you and a few dozen other SMBs. Since you are an SMB you are dealing with a third party customer service company. We cannot contact the division that has authorization to fix this part of the codebase. We have tried speaking to our Microsoft contacts but they are not allowed to contact the engineering team behind their product. Speaking from experience, this will only be fixed if someone with 10k seats has the same issue. You should close your account now. Sorry."
Never again.