P-Jones is right - disabling the SMTP proxy will stop sme from intercepting mail and you will send and receive mail using your isp for all the existing accounts.
Note this does mean that errant programs on your internet will also be able to send email without sme intercepting - ie outgoing mail using external smtp settings will simply travel through the sme.
In regards to the internal only mail this is easily achieved (in fact all functionality will already be working) if you have a different domain from your isp hosted in SME (if not you can add as many virtual domains as you like - the name will only work internally so you could call the domain anything - as long as you don't want to access the real domain from internal - again this is alterable, by separating www and mail records in your virtual domain settings).
If your sme domain name is the same as your external mail this would still work internally but you would have issues picking up the mail from your external accounts.
all users setup in sme for file sharing will automatically have a second email account from the external mail (or in the case where they are new and the sbs license is too expensive a single account provided by the sme).
There should be no problem internally sending and receiving to these accounts, and no cost. This could be setup on all internal mail clients with the username / password and the sme server details (even simply the internal ip of the server will work) for the pop3 and smtp or imap settings.
Alternatively you could use webmail -
https://yourserver/webmail. The horde client is excellent and there are a number of add-ons that you may find useful for internal only - such as file sharing, shared address books and shared bookmarks. Turn on the webmail in server-manager and check this out, it will already be setup for all your internal users - don't you just love this distro!. things like the automatic pseudonyms such as groupname@ and everyone@ may also be useful - ie email sent to everyone@yourlocaldomain will be received by all users.
The only downside would be problems associated with having two distinct accounts - ie - issues such as user confusion and having to check two accounts, also sending mail from the internal to the external accounts will require changes to the way the SME is setup (sending mail to any external client will be problematic if the domain is not dns registered and hosted on your sme), however this is also possible with tweaking if the ability to send externally is a must have.
Have a look at the manuals and / or fire more questions back here if you want more info, if you want more integration send more specific information about what you are trying to achieve, what is important.
Cheers, Shell