Why do some email users receive 401 errors when there is an image in the signature?

Question

When accessing the RoundCube webmail interface for some of the email users on my server, they encounter a 401 error or get logged out when composing and sending emails. What can I do to resolve this?

Answer

If a user has an email signature with an image, and this image is linked to an invalid cpsession, you will want to modify the signature and remove the invalid session ID link from that user's email address' signature.