Hi HectorG,
To extend on wyron's answer, you could also install something like Vincent Filali's multi-pop addon which will allow you to collect POP3 mails from quite a number of different mail accounts on a user by user basis.
Eg. User1 has 3 mail accounts out there (us1@ISP1.com, myaccount1@ISP2.com, bigdeal@isp3.com) and User2 has 2 mail accounts (us2@isp1.com, haha@isp4.com)
Using the multi-pop addon, your SME server could collect the mails from all those mail accounts and drop them into the approriate user's mailboxes.
Then, as suggested by wyron, you can use IMAP on your LAN stations to access the mails and webmail when you are out of the office.
Hope this helps.
Kelvin