Error: 'Cannot connect to SMTP server...' - Kerio Connect – GFI Support

Overview

Sometimes users report that their emails are not getting sent to external mail servers, domains, or recipients. The warning log shows dozens of entries similar to this:

[14/Jul/2019 14:50:43] Cannot connect to SMTP server <server_name>. Messages will stay in the message queue.

In such a situation, Message Queue in Kerio Connect Webadmin is full of emails with status 4.4.1 Cannot connect to host.

Requirements

Access to Kerio Connect server
Access to firewall settings

Root Cause

A firewall is blocking communication from the Kerio Connect server to External Mailservers.

Resolution

To resolve the connection issue:

Check MX records for Kerio Connect domain (using the MX Lookup Tool). The records should be pointing to the correct mail server. The first record should be starting from mail.connect_domain.com
Enable additional Debug logs.
1. Open Kerio Connect Webadmin.
2. Navigate to Logs > Debug, right-click on logs area, and click on Clear Log.
3. Right-click on logs and click on Messages.
4. Enable SMTP Server, SMTP client, and Queue processing.
5. Send a test email to the (desired) outside email address.
6. Gather Debug logs: Navigate to Logs > Debug, right-click on logs and click on Save Log.
7. Generate Support Information file by navigating to Status > System Health > Support Information.
Analyze Debug logs. You will see an output with messages similar to the below:

[14/Jul/2019 19:22:31][123145371508736] {qproc} SEND_REMOTE: Started delivery of queued message id=5d2be366-00000000 sender=<username@connect_domain.com> [14/Jul/2019 19:22:31][123145371508736] {qproc} The default MX delivery rule is applied for sender <username@connect_domain.com> and recipient <ext_user@external_domain.com> [14/Jul/2019 19:22:31][123145371508736] {qproc} Scheduling recipient: ext_user@external_domain.com to a batch in rule 'external_domain.com' [14/Jul/2019 19:22:31][123145371508736] {smtpc} Connecting to server mail.external_domain.com (<ip_address_of_external_server>:25) using local interface 0.0.0.0 ... [14/Jul/2019 19:23:45][123145371508736] {smtpc} Deliver mail: Cannot connect to SMTP server. User: , server: mail.external_domain.com [14/Jul/2019 19:23:45][123145371508736] {qproc} SEND_MX: Result for recipient ext_user@external_domain.com: delayed, Status: 4.4.1 Cannot connect to host
Try to telnet to external_domain.com.

telnet external_domain.com 25

The connection will be refused.
Check Firewall or Antivirus settings to allow traffic flowing from SMTP standard port (25) to the outside world (external servers).