prompt on sending mail "privateKey"

I just switched from Postbox back to Apple Mail (for reasons I won't go into here). The process seemed to go without a hitch, but now when I try to send an email I get a prompt (screenshot attached) which reads:

Mail wants to sign in using key "privateKey" in your keychain.

To allow this, enter the "login" keychain password.

1. I tried entering my current login password, my icloud password, and an old login password. No luck.

2. I restarted, and was able to log in with my current password.

3. I logged into iCloud with my iCloud password, no problem.

4. I read a lot of support articles both here and on other support sites, but none of them addressed Apple Mail specifically. Almost all of them had suggestions on what to do if your general login password stops working. This wasn't the case for me.

5. Nevertheless, I went into the keychain access screen and looked at settings for iCloud and for login. I didn't see a setting specifically for Apple Mail. Every certificate I looked at indicated "always allow for applications" and the 'edit' button was grayed out. Sometimes the dreaded 'unspecified error' shows up, sometimes I am told I don't have access. Mine is the only account on this mac, and I am the only administrator.

In summary, this problem is restricted to Apple Mail and I'm going around in circles. Can anybody point me in the right direction? Many thanks in advance.