Thanks, Brian and Guy.

I found the option to 'Remember and apply each folder's sorting mode separately'.  It was ticked, so I unticked it, thinking that I could then have them all sort by date.  Then I went into one of the folders that was correct (sorting by date), and it was still by date.  I then went into one of the problem folders and it was still by sender, but this time, when I clicked on sort by date, closed it and reopened it, it had kept the sort by date.

The folders now seem to be behaving normally - which I find a bit odd.  I'd expect that, having unchecked the 'remember and apply...' they would now all have the same sorting, but they don't!

Guy - under grouped views each had none (normal view) ticked?

They are now doing what I want them to do, so thank you.

In the past week or so, several of my folders default to sorting themselves by sender.  No matter how I leave them, next time I go into them they are sorted by sender.  I found a thread by davidc called "Default Sorting problem in CopySelf folder", where he mentions that "Options is still set to remember the sort of the last view".  I can't find anything like that in the Options - what am I missing?  I also can't see anything wrong with the headers, which seemed to be davidc's problem.

One of the folders has 794 messages, another 506, and another 272, so it doesn't seem to be linked to size of the folder.

Anne

This is the setting that controls separate folder sorting mode is called "Remember and apply each folder's sorting mode separately" and is located in Tools | Options | Incoming mail | Preview mode settings:

 Is this mailbox being accessed by anyone else?  I ask because I have a mailbox that is checked by one of my co-workers when I am out of the office.  I thought there was a Pegasus Mail sorting issue only to discover that my co-worker was resorting to his liking.

[quote user="bfluet"]
This is the setting that controls separate folder sorting mode is called "Remember and apply each folder's sorting mode separately" and is located in Tools | Options | Incoming mail | Preview mode settings:
[/quote]

Then open each folder that is misbehaving and look at the
Sort by... and Grouped views
options in the Folder menu.