Server error after sending mail and Display mailboxes

I have successfully set up an e-mail account. Mail is received normally (list) and mail can be sent. “Server error” will be displayed after sending.

Below is the Nextcloud error log.

  • (mail) Saving draft failed: IMAP error reported by server.
  • (PHP) fsockopen(): unable to connect to MAIL_DOMAIN:993 (Connection timed out) at /var/www/vhosts/NEXTCLOUD/apps/mail/lib/Service/AutoConfig/ConnectivityTester.php#45

For the display of the mail application, the leftmost is the account and the inbox, favorite column. The middle list is the list of received mails. The right is the mail body.
Depending on the mailbox size of the mail account, the display of the mail application is slow or an error display. If the size of the mailbox is small, it is displayed quickly, but if it is large it is hard to display. And “We could not load inbox” is displayed. In case of this error, clicking “mail address” in the leftmost column displays the received mail.

I feel this behavior abnormally first.
The next question to wonder is “display items in the leftmost column”.
It is now “mail address”, “inbox”, “favorite”, and “light folder” in light characters. Can I display “Sent” or “My create folder” on the IMAP server? Should this normally be displayed properly? Is it only my environment that they are not displayed?

Is there a way to improve this?

OS: CentOS 7.3
PHP: 7.1.7, nginx 1.11.10
Nextcloud: 12.0.0 and 12.0.1
Mail app ver. : 0.6.4