1. 22:45:23/200107(..mail/certs)% grep imap4 /usr/pkg/etc/imapd.conf

   tls_cert_file: /usr/pkg/etc/letsencrypt/live/imap4.example.com/fullchain.pem
   tls_ca_file:   /usr/pkg/etc/letsencrypt/live/imap4.example.com/fullchain.pem
   tls_key_file:  /usr/pkg/etc/letsencrypt/live/imap4.example.com/full+priv.pem
   

2. 22:45:42/200107(..mail/certs)% grep pem /etc/mail/sendmail.cf

   O CACertFile=/etc/mail/certs/cert.pem
   O ServerCertFile=/etc/mail/certs/cert.pem
   O ServerKeyFile=/etc/mail/certs/privkey.pem
   O ClientCertFile=/etc/mail/certs/cert.pem
   O ClientKeyFile=/etc/mail/certs/privkey.pem
   # dnl please make keys named certs.pem and privkey.pem in /etc/mail/certs.
   # define(`confCACERT', `CERT_DIR/cert.pem')
   # define(`confSERVER_CERT', `CERT_DIR/cert.pem')
   # define(`confSERVER_KEY', `CERT_DIR/privkey.pem')
   # define(`confCLIENT_CERT', `CERT_DIR/cert.pem')
   # define(`confCLIENT_KEY', `CERT_DIR/privkey.pem')
   

one more command is necessary after certbot update

2020-01-05 07:57  cat fullchain.pem privkey.pem > full+priv.pem

imap4@root 08:01:10/200105(..pkg/etc)% ls -lLR letsencrypt/live 
total 4
drwxr-xr-x  2 root  wheel  512 Jan  5 07:57 imap4.example.com

letsencrypt/live/imap4.example.com:
total 44
-rw-r--r--  1 root  wheel  1903 Dec 18 23:35 cert.pem
-rw-r--r--  1 root  wheel  1647 Dec 18 23:35 chain.pem
-rw-r--r--  1 root  wheel  5254 Jan  5 07:57 full+priv.pem
-rw-r--r--  1 root  wheel  5197 Dec 18 23:54 full+priv.pem-5197  (old and bad one)
-rw-r--r--  1 root  wheel  3550 Dec 18 23:35 fullchain.pem
-rw-------  1 root  wheel  1704 Dec 18 23:35 privkey.pem

CF-SX3@makoto 07:17:40/200105(~)% openssl x509 -text -noout -in <(openssl s_client -connect imap4.example.com:993 ) | grep -A3 Validity

depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
verify return:1
depth=0 CN = imap4.example.com
verify return:1
DONE
        Validity
            Not Before: Dec 18 13:35:18 2019 GMT
            Not After : Mar 17 13:35:18 2020 GMT
        Subject: CN = imap4.example.com
CF-SX3@makoto 08:00:03/200105(~)% 

587 port needs different command to check the valid thru date

22:42:41/200107(~)% openssl s_client -connect imap4.example.com:587 -starttls smtp | openssl x509 -noout -dates         
depth=0 CN = imap4.example.com
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 CN = imap4.example.com
verify error:num=21:unable to verify the first certificate
verify return:1
250 HELP
notBefore=Dec 18 13:35:18 2019 GMT
notAfter=Mar 17 13:35:18 2020 GMT