I can confirm part of your issue.
Sort is fine by subject or date - however removing the 'received' column hides mail.
5.0.6_GA_2313.UBUNTU6
5.0.6_GA_2313.RHEL4_64
Convo or message view.
Works fine in 5.0.5_GA_2201.RHEL4.64
Infact removing/unchecking the received column hides mail everywhere else so I filed:
Bug 28381 - unchecking received column hides mail
Could one of you file for the sent folder sort by date when trying to sort subject issue?
Before someone asks-
The To column in the sent folder is in a world of itself:
Bug 6830 - sorting Sent folder by To column sorts wrong Bug 12734 - Message view + in:sent To column header allows sort
And From column when in other folders in convo view:
Bug 7972 - Mail sort using from column not enabled