Most email systems have a spam filter to stop your mailbox from filling up with ten tonnes of crap about Viagra and degrees you can buy from the university of Some Bloke's Garage. They tend to look for either keywords or particular characteristics of the email (who it was sent from, or were you blind copied on it along with 3 billion other people).
If you've emailed your work address from home before, either the filters have been changed on your work email server recently or you've tripped the system with a keyword. Things like exclamation marks in the subject or the word 'Free' can do this.
You should be able to right click on the mail and mark your home address as a safe sender or have an option 'never move to Spam' or similar.
It's also possible that at work the spam filtering may now include more of the free mail services, so Hotmail / Outlook / Live.co.uk (all owned by Microsoft), GMail (gmail.com, googlemail.com from Google), and Yahoo.co.uk ( Yahoo! Mail, which also serves some ISPs), could all have been added as they are commonly used as 'From' addresses for spam, even if the spam is sent from somewhere in China with no link to the named mail service.