Today, I installed Thunderbird 3.0 on may Mac to check out the features that have been added. The experience of installing and setting everything up was pretty good (just drag and drop).
But the surprise happened right away during writing an email where I wanted to send a PDF to a friend. I wrote „Attachment“ and Thunderbird instantly showed a message of the bottom of the Window to remind me of the „Attachment“ that I was about to sent (and which shouldn‘t be missed in the mail).
Perhaps this is a feature that other E-Mail clients have since decades, but it‘s the first time I saw this really useful feature. I looked into the preferences and there is a section where you can specify „keywords“ whose appearance will bring up this reminder to you. So if you prefer to name the Attachment differently, then you can specify something else here.
BTW: The reminder appears only if there is nothing already attached.