[redacted]: but they DO get copied to the sent folder
[redacted]: thunderbird just doesn't know aobut it
Finally, realizing the main reason I was using Evolution was to get things done, I switched back to Mozilla Thunderbird. Since it has cool cutting-edge features while being relatively stable, I decided to use Thunderbird 1.5 Release Candidate 1, as opposed to the older 1.0.7.
I was mildly annoyed to find that every single time I attempted to send a message, Thunderbird failed to append it to the Sent folder. While it was still better than Evolution (in terms of both stability and features), it was still sub-optimal.
Luckily, this page from the University of Wisconsin Help Desk came to the rescue! After following the instructions, I can send messages with impunity, and without having to worry that this time (unlike all of the others), the Thunderbird error dialog is actually correct and the message has not actually been saved to the Sent mail folder.
 Compared to Novell Evolution, Mozilla Thunderbird is freaking Gibraltar.