[Dovecot] problems with ssl cert

in conf.d/10-ssl.conf:

ssl = yes ssl_cert = </etc/ssl/certs/ms1xxx.net.crt ssl_key = </etc/ssl/private/ms1.xxx.net.key

when try to set up an account in thrunderbird, get /var/log/mail.log:

imap-login: Disconnected (no auth attempts): rip=223.205.150.234, lip=xx.xx.xx.xx, TLS: SSL_read() failed: error:14094416:SSL routines:SSL3_READ_BYTES:sslv3 alert certificate unknown: SSL alert number

openssl s_client -connect mail.myserver.com:993 - successful

using the same cert in gning / chrome is successful, so think the cert is ok