Some customers have reported problems accessing their individual (domain aliased) webmail client using bookmarks.
There have been some general security updates as well as recent issues with DNS aliases which can cause a number of problems.
In either case, generally the resolution is to update the bookmark and remove the /imp/blah/blah.php from the end of the link so that it looks like http://webmail.domain.com/
Alternatively you might like to try the improve @mail webmail interface at https://webmail.fido.net/
Fido’s Pro Webmail service utilises the latest version of AtMail (@mail) which includes calendaring, contacts and ActiveSync support (for push email to iPhones/Blackberrys/etc)
More information on the Fido Pro Webmail service can be found online at http://www.fido.net/webmail/