Unable to use encrypted password for imap and pop3

[Davide Marchi]

Hi friends,
Following the Workaround tutorial for Jessie 
(https://workaround.org/ispmail/jessie ) I've set up six month ago my 
first Postfix email server and all works very fine, except for the 
ability from the user to encrypt login password.
I can use the encrypted login password with smtp (Postfix), but not 
with pop3 and imap (Dovecot).
The certificates are from Letsencrypt.

I've read 
"http://wiki2.dovecot.org/Authentication/Mechanisms#Non-plaintext_authentication" 
and set

auth_mechanisms = plain login cram-md5


But with no luck,

If I try to connect to Dovecot from openssl obtain an error:

"Verify return code: 20 (unable to get local issuer certificate)"

And now, if I enable on the client (Tunderbird) the encrypted password 
obtain a message to reinsert the password

Here my Dovecot configuration:

doveconf -n
# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.16.0-4-amd64 x86_64 Debian 8.5 ext4
auth_mechanisms = plain login cram-md5 apop
mail_location = maildir:/var/vmail/%d/%n/Maildir
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope 
encoded-character vacation subaddress comparator-i;ascii-numeric 
relational regex imap4flags copy include variables body enotify 
environment mailbox date ihave
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     special_use = \Drafts
   }
   mailbox INBOX.Junk {
     auto = subscribe
     special_use = \Junk
   }
   mailbox INBOX.Trash {
     auto = subscribe
     special_use = \Trash
   }
   mailbox Junk {
     special_use = \Junk
   }
   mailbox Sent {
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     special_use = \Sent
   }
   mailbox Trash {
     special_use = \Trash
   }
   prefix =
}
passdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
   driver = sql
}
plugin {
   sieve = ~/.dovecot.sieve
   sieve_after = /etc/dovecot/sieve-after
   sieve_dir = ~/sieve
}
protocols = " imap lmtp sieve pop3"
service auth {
   unix_listener /var/spool/postfix/private/auth {
     group = postfix
     mode = 0660
     user = postfix
   }
}
service lmtp {
   unix_listener /var/spool/postfix/private/dovecot-lmtp {
     group = postfix
     mode = 0600
     user = postfix
   }
}
ssl_cert = </etc/letsencrypt/live/server.sio4.org/fullchain.pem
ssl_key = </etc/letsencrypt/live/server.sio4.org/privkey.pem
userdb {
   args = uid=vmail gid=vmail home=/var/vmail/%d/%n
   driver = static
}
protocol lmtp {
   mail_plugins = " sieve"
}

Could you suggest me where I should look into?

Many many thanks!


davide