As far as I know there is no way to sort the mailbox index by more than one key. It would be a good feature to have. It is not something that could be automated via scripting either but there is one way to achieve this provided you are only wanting to look at a single sender:

1. Right click on a message from the sender in the message index
2. From the popup menu select "Show only this sender"
3. Click the column tabs to sort the messages as you require.

Note: You can also use this technique to filter messages by other criteria and then sort them. At the top right of the message index you will see a "Show ..." text.; Usually this says "Show all mail". If you click on the "all mail" text you will see a list of choices these can be used to filter the messages displayed in the mailbox (the other messages are temporarily hidden from view, they are not deleted).

Scripts can work in conjunction with the "tagged" entry. It is possible to write a script that tags messages based on any set of criteria you require. This includes multiple criteria such as "all unread messages between 01July2001 and 31Aug2001 that do not have attachments". An example of a script that does this is the "TagAnnotatedMessages" script I wrote (available in the registered users area).

One other comment on sorting messages. At the moment sorting is not persistent, that is Poco does not remember the sort order you used from invocation to invocation. This is a feature that has been requested by several people. Hopefully Slaven will implement this in a future version. Do not expect to see it in the next public release which is solely to deal with problems reported in Poco 2.50. When Slaven does implement this I would love to see the ability to sort by multiple fields in the index, it's a great idea!