"Why are my domains being blocked by Smartfilter"
To put it as simply as possible, smartfilter categorized your site as (for example) "Sports and recreation" your company decided to block all sites in the category "Sports and recreation"
Your site isn't being directly blocked, it just happens to sit in a category your company chooses to block.
To fix this you can either challenge your workplace to unblock that category, or challenge smartfilter that the categorisation is an incorrect one. Without knowing what your website is, or how Smartfilter deals with challenges, I don't know how successful either is likely to be.
You won't be able to this via your own webhost or registrar - if you were allowed to do this it would be wide open to cheating and gaming.
"How can domains be categorized?"
From my observational experience categorisation is fairly simple - sites are crawled and if the content matches some keywords in a category it goes in there.