jpeg attachment

I have a client who is unable to send jpeg attachments using Outlook or even outlook express. The server always times out or says “Unable to log onto outgoing mail server” etc.
Other attachments work fine and size is not the issue. I have uninstalled outlook and outlook express and reinstalled but this did not fix the problem.
I have also tried creating new profiles and then them sending but get the same result.
Also I have set up another isp and email account but get the same result.
I have downloaded a program which changes outlooks security settings but did not find jpeg as a blocked attachment.

Any suggestions?