Emclient is asking for authentication when I read emails from certain contacts

When I read email from certain contacts, emclient ask me for username and password for the remote host (zimbra) managing emails for these contacts.
I think, emclient is trying to download some content from the external server which ask for authentication.
Its only for some contact all manage by the same provider which use zimbra as mail server.
When I clic on cancel, I can read the mail…
It is really annoying because I receive a lot of emails from these contacts.
Please help. Thanks.