To type an email adress into something you have to use an email client, let it be firefox, safari, mail, or some other client you are reading it literally. Ofcourse i'm talking about the email adress field.
When you manually type in more than one email address in the To: field of MSOutlook, you have to use a semicolon to separate the addresses. There are also no spaces between the addresses. Using my Yahoo email account in Internet Explorer, it works exactly the same.
Apple (on the iPad and iPhone?)uses only a space between addresses and highlights each address as you type the addresses. What is interesting is that when you hit Return or the Comma it assumes that you have finished typing the address, jumps a space and highlights what you have typed. If however you hit the spacebar, it does not recognise that you have finished typing the address and it includes the space in the address you have typed when you press Return. When you go to the subject field, a comma (and the space) automatically appears between each address in the address fields.