[imp] What causes "Linking from untrusted source is denied!"

[Joonas Hämäläinen]

Hips,

I have encountered problem with https:// links in imp read mails. When 
one receives mail which contains link to file, and that is https:// 
link, imp will be throwing error:

Linking from untrusted source is denied!
webmail.yourdomain.com

Our horde installation is behind SSL, target is behind SSL certificate 
and both certificates are valid thawte certis. If I then use our testing 
horde installation which runs over regular http, and read similar mail, 
I can open this file just fine.

What causes this error, Linking from untrusted source is denied? If I 
refresh that page, it loads that file, but why it gives this error 
first? And how do I remove this step to make it easier for end-users to 
open their files.

And to make it clear, problem is with files. Like 
https://www.example.org/folder/example.pdf written to mail, and that is 
generated as link and causes this error.

Joonas Hämäläinen