UnTrusted Referer warning after automatic mailbox refresh

When a user sets their "Seconds between automatic mailbox refreshes" to a value, and the mailbox refreshes at the correct interval, the user gets the WARNING screen about untrusted referer.

I set the general WEBMIN preference to allow untrusted referers to fix this, but I'd rather not do that. I assume there is some security risk with that option.

thanks...
Steve