You can have as many domains/users as you want. (Unless it's like five billion and breaks the service or something.) Generally phone access is third party thro

You can have as many domains/users as you want. (Unless it's like five billion and breaks the service or something.) Generally phone access is third party through IMAP. On Android I personally use K-9 mail, but you can use anything that supports IMAP anywhere, which is a pretty good number of options for any platform.