Hi Tom,
This may or may not apply to you as I skipped SME 5.5 altogether and moved from 5.1.2 to 5.6.
I identified a problem with bind (that does DNS lookups for your SME server) in SME 5.1.2 in which the server will not resolve some domain names. Hardware routers, W2K servers, direct dial to the ISP, etc. all works but not if you use SME 5.1.2 as the DNS server. I fixed the problem (the link to the howto is posted at contribs.org) by upgrading bind in SME 5.1.2 to the version used by SME 5.6 and the domains will resolve correctly after that.
Again, because I totally skipped SME 5.5, this may not apply to you, but thought I'd mentioned it in case it might help.
Kelvin