For the benefit of list, I've decided to work-around the problem using: imapc_ssl_verify = no
Obviously I still welcome suggestions as to how I can get dsync working with Let's Encrypt certificates and when OpenSSL validates "ok" but Dovecot does not (despite Dovecot supposedly falling-back to OpenSSL). For the record, I have done this sort of dsync before (i.e. "dsync backup" from source that has Let's Encrypt cert), I've never had a problem before, so I'm wondering if it's something peculiar to Dovecot 2.3.17.1 (whether a bug or a feature, it would be nice to know what's changed since I would have thought this sort of scenario should work "out of the box").
