If a remote Webmail user is using an Internet connection that uses a transparent Proxy Server the IP address that user is assigned by the ISP and therefore the one MDaemon sees connecting, may change throughout the Webmail session.
If your Webmail configuration requires the remote users IP address to remain the same during a Webmail session (this is the default setting) this will result in an Invalid IP Session error when that remote user attempts to login. This option is more secure as it ensures nothing else can ‘steal’ the active users session and so is recommended.
However, to allow this remote user to login when using this Internet Connection you would instead need to untick the Require IP persistence throughout Webmail session option at Setup -> Web & IM Services -> Webmail -> Web Server:
