OS: Ubuntu 20.04.2 (on mutli-core VM)
Dovecot (Ubuntu default/repo version):  2.3.7.2 (3c910f64b)
OpenSSL (Ubuntu default/repo version):  1.1.1f  31 Mar 2020

Reproducing-

Run:  "openssl s_client -showcerts -connect imap.example.com:993 -servername imap.example.com" (using a diff domain obviously)

Produces error: "Verify return code: 21 (unable to verify the first certificate)" (Meaning it is missing a CA verify from what I understand?)

The "Verify return code: 21" error ONLY came to my attention after I had a customer complain about adding an email account to an Android phone (using the Google/Gmail default email app). -> "Certificate cannot be trusted" was shown by app when verifying imap connection. I could force it to be used, but this still bothered me, obviously. *The same certificate bundle is also used by smtp/postifx and www/nginx and works just fine. Also the openssl test shows 'verified' on both*

I am posting to list mainly because on an older version of Dovecot I have running (default repo version for Ubuntu 18), I do not have this problem shown during testing with openssl. I did not have to change the ssl_cert or ssl_ca value in config. It has identical local.conf settings other than that. Granted it is also an older openssl version, too. So, I feel this may be a bug with Dovecot (or possibly OpenSSL).

I finally 'fixed' it myself by using the LE 'fullchain.pem' certificate as the location for the 'ssl_cert' entry (and chain.pem for the ca entry). Previously, it was using the normal cert.pem file location. This is still the way it's setup on the other older machine and still works fine. Changes-

|ssl_ca = </etc/letsencrypt/live/{CertName}/chain.pem (or 'fullchain.pem' should work) *ssl_cert = </etc/letsencrypt/live/{CertName}/fullchain.pem* (was 'cert.pem' previously) ssl_key = </etc/letsencrypt/live/{CertName}/privkey.pem|

You can also view the problem here for more info : https://superuser.com/a/1640778/47628

Thanks ahead for any insight into this..