And as usually happens, right after posting this I found the problem by
looking in the right place!:
Oct 01 17:06:58 Error: pop3-login: Failed to initialize SSL server
context: Can't load DH parameters: error:1408518A:SSL
routines:ssl3_ctx_ctrl:dh key too small: user=<>, rip=199.73.114.98,
li
Just applied Debian SSL updates, due to vulnerabilities reported:
"Package: openssl
CVE ID : CVE-2019-1547 CVE-2019-1549 CVE-2019-1563
Three security issues were discovered in OpenSSL: A timing attack
against
ECDSA, a padding oracle in PKCS7_dataDecode() and
CMS_decrypt_set1_pk
