I finally decided to switch to Thunderbird. Having the gecko HTML engine is handy, and I have found ways to work around most of the annoyances.
Add-ons:
- Attachment Reminder
- British English dictionary
- keyconfig
- Opening Words
- Quote Colours
- SmoothWheel (AMO)
Note that Quote Colours does not work with 3b4 at the moment.
Change the delete key in keyconfig to “MsgMarkMsgAsRead(true); goDoCommand('cmd_delete');” so that deleted messages are automatically marked as read.
Set mailnews.threadpane_select_delay to 0.