The user with a mailbox on the Kerio Connect server sends an email to external email address. The message is being rejected by the Kerio server because the user cannot relay through this server. The user is configured for SMTP Authentication in Outlook. The user had no other issues sending messages.
Upon examining the log file, the mail server couldn't resolve the DNS name.
Status: 5.5.4 Cannot find hostname "external_domain.com" in DNS, Remote-Host:
Access to the Kerio Connect Administration
Kerio Connect has a default retry interval for SMTP messages. It can be set up in Configuration -> SMTP server -> Queue Options. The default value is set to 30 minutes.
The same value is configured for POP3 before SMTP (if enabled).
If the email delivery was failed, Kerio Connect will retry resending the message after 30 minutes.
[13/May/2020 07:06:32] Sent: Queue-ID: 5ebbd4b5-00003add, Recipient: <user1@external_domain.com>, Result: delayed, Status: 5.5.4 Cannot find hostname "external_domain.com" in DNS, Remote-Host: external_domain.com, Msg-Id: <email@example.com>
[13/May/2020 07:36:33] Sent: Queue-ID: 5ebbd4b5-00003add, Recipient: <user1@external_domain.com>, Result: failed, Status: 5.1.1 550-(mail.kerio_domain.com) [18.104.22.168]:47903 is currently not permitted to
[13/May/2020 07:36:33] 550-relay through this server. Perhaps you have not logged into the pop/imap
[13/May/2020 07:36:33] 550-server in the last 30 minutes or do not have SMTP Authentication turned on
[13/May/2020 07:36:33] 550 in your email client., Remote-Host: external_domain.com, Msg-Id: <firstname.lastname@example.org>
This is the default behavior of Kerio Connect.
The initial "Cannot find hostname in DNS" error may occur because of External domain issues or local Outlook problems.
Resend the necessary emails from Outlook after some time. If the issue is still happening, reach out to the external domain user using alternative methods (chat, call, meeting, etc).