If you are using outlook express or similar means to send/receive, the "my server requires authentication" box needs to be ticked. If it is not ticked it will not send mail, but it will still receive. To find this setting open o/e click on tools,accounts,highlight the mail account,click on the "servers" tab and the box referred to is at the bottom of the frame on the left. Tick it and then click Apply.Then you are done, exit out of all of the pages and try it, see if it sorts it.
Chris F, April 2008