As a note, SME is designed for Small to Medium Enterprises, 10-100 type people as an example. Most busineses of this size do not have an IT staff, they have someone that "knows about computers". This distro allows a reseller to install into that base, train the person that "knows about computers" how to add users, check logs and various basic things within the server manager. If said SME has a trouble of major proportions or need something done, they call their reseller, who will try to customize it themselves. Not too many end users actually puruse these forums looking for answers.
Service Link allows the end user/reseller to have a purchase/updated, supported anti virus solution, point and click IPSec VPN setup (by the reseller, not the end user) and a few other things. Most things offered with Service Link can be done for free as an add-on, it just won't always work, is harder to configure and is not supported by Mitel's Product Support team.
In mention of not being able to configure squid and firewall rules; it's not offered in the web manager because the guy that "knows about computers" will have no clue as to what squid, nat or a proxy even is, he's a Windows guy don't ya know! It's the resellers responisiblity of making configuration changes (if needed) and doing it properly (via the templates). Yes, there are some people on here who use this as a company server and support it themselves, but they are not the normal profile as much as we'd like to believe.
The default settings in a default install work quite fine for Squid and NAT, it routes just fine, with add-ons like SquidGuard and an open source mail scanner it's secure and safe.
If samba/squid/nat'ing doesn't work for you out of the box, you've done something wrong or don't have supported hardware. As for your problem of getting to Hotmail, Yahoo or whatever other mail websites, I've never had or seen this trouble and I maintain 4 of these boxes, 3 are dev releases (5.5) and 1 is a Service Link enabled copy (also 5.5).
Hope this clears up some of your questions/problems.
Terry