I used CSS in my email and sent it out.
When I received the email in Gmail, all the CSS was disabled; however, when I retrieved the email in thunderbird or outlook, everything is OK.
How would I fix this?
Gmail doesn't block CSS totally. It still supports inline style. Why doesnt it support style block, I've no idea. To have a good overview of CSS support in various webmail and email clients, here's a good one. http://www.campaignmonitor.com/css/