O365
The Basic (lowest tier) Business account is roughly same price as the personal 365 account, but I believe they are completely different services. If you have an existing personal 365 account, I don't believe there is a seamless switchover to a business account. You will need to create a business account separately.
I haven't actually switched it yet so I can't confirm that it works, but the option is there.
I'm just trying to figure out if you can add multiple domains and aliases (that can be used for sending) with a single user license.