Outlook 2007 marks my mail sent as spam

I have a number of accounts on mydomain.co.uk that are configured as "gmail with a custom domain". It's been like this for a number of years.
Since last Thursday, I noticed that outgoing e-mails sent from xxx@mydomain.com via Outlook 2007 were in junk mail folders. This occurs for all accounts. Mail sent from these addresses by phone (gmail) or web (again gmail) are not affected.
Incoming emails are not affected.
Indices to try?