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Gmail on Android said the certificate is invalid (Dovecot SSL)

Issue

Gmail on Android reports that the certificate is invalid when configuring an IMAP connection to the server.

Checks

Check on server 

# dovecot -a | grep ssl
auth_ssl_require_client_cert = no
auth_ssl_username_from_cert = no
imapc_ssl = no
imapc_ssl_verify = yes
pop3c_ssl = no
pop3c_ssl_verify = yes
    ssl = no
    ssl = no
    ssl = yes
    ssl = no
    ssl = yes
service ssl-params {
  executable = ssl-params
  unix_listener login/ssl-params {
  unix_listener ssl-params {
ssl = yes
ssl_ca = 
ssl_cert =

Check from client 

# openssl s_client -connect mail.domain.net:imaps
# openssl s_client -connect mail.domain.net:pop3s

Answer 

Root certificates are not specified: the ssl_ca parameter is empty.