trusting a cert

[Leo Butler]

How can I get gnus/gnutls-cli to trust an imap server's cert? Some
recent changes in that pipeline, or debian's list of trusted issuers,
have made one of my imap servers inaccessible from gnus.

Leo

<-----

*** Fatal error: Error in the certificate.
*** handshake has failed: Error in the certificate.
Processed 166 CA certificate(s).
Resolving 'wm.sdf.org:993'...
Connecting to '205.166.94.23:993'...
- Certificate type: X.509
- Got a certificate list of 1 certificates.
- Certificate[0] info:
 - subject `CN=wm.sdf.org', issuer `CN=thawte DV SSL CA - G2,OU=Domain
 Validated SSL,O=thawte\, Inc.,C=US', serial
 0x094088c8022c507f2d425c99095860ab, RSA key 2048 bits, signed using
 RSA-SHA256, activated `2015- 04-19 00:00:00 UTC', expires `2018-05-18
 23:59:59 UTC', key-ID
 `sha256:8f07403975f9ddb4ba778b268462b4dc74e4e3ca87f13c85552f16ab34c3312a'
         Public Key ID:
                         sha1:a2b595a62a8d208240a1fc8adf6027fd625781fa

sha256:8f07403975f9ddb4ba778b268462b4dc74e4e3ca87f13c85552f16ab34c3312a
        Public key's random art:
                        +--[ RSA 2048]----+
                                        | ..              |
                                                        |o.               |
                                                                        |o.
        .         |
                        |. .   . ..       |
                                        |o  . .o S.       |
                                                        |* ...o
        *.        |
                        |+o+o+.o.         |
                                        | oo=+oE          |
                                                        |
        .ooo.          |
                        +-----------------+

- Status: The certificate is NOT trusted. The certificate issuer is
unknown.
*** PKI verification of server certificate failed...