Apple mail error message, not sure what the icon means.

Im continuously getting an error message/alert top right of my screen (with @ in blue icon) not sure what this icon means?

error https://cp16574.hpdns.net:2080/rpc/principals/ my email address

although my email seems to be working fine? I just keep canceling it as when I try to put a password in, it wont recognise it. But it happens 2 or 3 times a day so is annoying.

Any help appreciated.

[Edited by Moderator]