I might give Courier a try - I got a free license when they were giving it away a while back and never got round to trying it. (It is still available for free at http://www.rosecitysoftware.com/courier/
so it is worth a look)
There is a full range of filtering options including RegEx.
One of the nice thing is the use of 'templates' for email accounts so that if you have, say , multiple yahoo addresses you can set up a single email account/server set and then use it as a template to quickly enter all the other account details without having to retype all the server details. You can display full headers in the preview window (or an abbreviated version or none at all).
The only things I don't like are:
- the look ... on my screen the icons are tiny (even when set to large) and are 'fuzzy' so it is difficult to see what you are doing. The style is also quite dated.
- it is a bit convoluted to set up (but easy to find your way around once you have played for a few minutes)
- importing is a bit limited (it doesn't work with Outlook, but does with Outlook Express)
- no export function (biggy). You can archive your emails to a text file and a folder of attachments (inc. all the HTML images etc) which can be imported again, but I can't see how you could use this archive in any other app easily.
I have attached a screen shot ...