That PGP leak paper suggests that neither Mutt nor Claws are obviously insecure. I’ll give them both a go.

UPDATE: Well, apologies to the Outlook and Mail teams. I installed Claws, and it promptly told me I had a firewall configuration error. I fixed the error, and Claws and Outlook (and I presume Mail) worked. Outlook’s problem was far less serious that I supposed: rather than lying, it had simply refused to tell me what had gone wrong. It’s still annoying—it’s difficult to diagnose & fix a problem when software conceals the symptoms—but that’s a problem of design incompetence, not outright lying. Anyway, the fact is I got it wrong, Outlook does not lie, it’s just childish. Bah! Double Bah! Meh!