Mail Images not sending 1.5.5.1?

Hi everyone,

In Sales > Mail when I send an email with an image inserted, the email itself sends fine, but the images do not get sent.

I have tried in yahoo, google and outlook, and it is not my email provider/spam settings.

When I open the email it tries to load the images, fails, and reverts to the alternative text. When I try to view the image info in the email I get a 404 server error.

I have checked that all the pathways to the images are correct and use the long URL form when inserting the image.

Every other aspect of the mail system appears to work fine, it's just the images that are not sending.

Any advice or known fixes?

Aaron