A long-standing problem I’ve had w/ Mail.app was the continuing confirmation dialogs I’d get even after I thought I’d manually added the certs into my X509Anchors. Since 10.3, Apple has had an easier way [illustrated], but like for many people, doing the option-drag (or any kind of drag) would immediately lock up Mail.app.

Finally, I got a breakthrough. A comment here saying that the second server confirmation would work, but the first would cause lockups led me to experiment (since I have different self-signed certs for my IMAP and SMTP servers) a bit, and I’ve found a general solution that works w/ no lockups.

The secret to being able to successfully drag and drop is to cancel the initial (on-load) security dialog, and then to go back online. It seems that any certificates you try to drag immediately on startup will cause a lockup, but after that (after mailboxes have loaded?) it works fine.