looks complicated for renewal.
I solved by building a certificate for mail server purpose in standalone mode, seperated from other regular https certificates, like this:
root@ns1:/opt/letsencrypt# ./letsencrypt-auto certonly --standalone --preferred-challenges http -d mail.webologix.com -d smtp.webologix.com -d mail.mon-voyage-a-cuba.com -d smtp.mon-voyage-a-cuba.com -d mail.php-web-service.net -d smtp.php-web-service.net
Thanks for your help