There's two ways I know of to do this. As you've learned, separators do not work (semi colons for example). Mail merge reads the entire cell, the semi colon will be treated as part of the email address.
So how do we get around this.
Easy, in the excel data source, create a field titles emails or whatever. And put each email in its own cell below it.
When merging, select send e-mail messages. You will have to select the field "email messages".
Alternatively, you can set up mailing lists (I actually prefer this method), in Outlook. I'm currently using 2010, so I'll have to link you to the specific instructions:http://www2.etown.edu/docs/ITS/Tips-Outlook/OL2007-Distribution%20list%20as%20a%20data%20source%20for%20mail%20merge%20to%20email.pdf
It'll allow you to keep it more visual/organized in Outlook. I know it seems a bit overwhelming at first, but once you do it once you'll get the gist of it. Just remember, to my knowledge, there is no separator available to put emails in one cell, it's how it reads the cell that causes the issue. This applies to ALL versions of office.
Let me know if I misunderstood the question. If everything works out and once you're completely satisfied, please remember to return and rate my answer. If not, or you need some clarification/assistance, don't hesitate to ask via the REPLY