However I have to admit that my long-used and much loved KMail was not up to the task, for some unknown reason. My main gripe with it was that reading, moving and deleting messages took forever, and refreshing folders was a royal pain. I thought it was an unavoidable problem, related to the way IMAP works. However, I decided to give other e-mail clients a chance, and that I did.
I have installed and run Thunderbird (actually, Icedove, the Debian version of TB), and so far it's given me good impressions. All the slowness I suffered with KMail is gone, and I have to say this fact alone is driving me from KM to TB.
As an additional piece of info, here's how to set "plain text" as default for sending messages. Why would you want that? Read these tips. The reason why TB has no obvious button to set "all plain text" or "all HTML" escapes me, but that seems to be the case. However, there is an "advanced" mode of doing it: go to Edit->Preferences->Advanced->Config Editor. From the variable/value list there, you have to set (by double-clicking, for example) "mail.identity.default.compose_html" to "false". I also set "mail.html_compose" to "false", but the important variable seems to be the first of the two. From that point on, all the new messages you compose will be plain text.