I have zabbix server installation on ipv6-enabled host. Email notifications are configured to send email to localhost. But mailserver (exim4) is configured only on ipv4 addresses. Zabbix complains that it cannot send email to localhost:
> 30283:20140806:112635.337 In execute_action(): alertid  mediatype 
> 30283:20140806:112635.337 In send_email() smtp_server:'localhost'
> 30283:20140806:112635.337 cannot connect to SMTP server [localhost]: cannot connect to [[localhost]:25]:  Connection refused
> 30283:20140806:112635.337 End of send_email():FAIL
> 30283:20140806:112635.337 End of execute_action():FAIL
> 30283:20140806:112635.337 error sending alert ID 
At the same time telnet works without problem:
> telnet localhost 25
> Trying ::1...
> Trying 127.0.0.1...
> Connected to localhost.
> Escape character is '^]'.
> 220 test-srv211 ESMTP Exim 4.80 Wed, 06 Aug 2014 11:33:41 +0600
> 221 test-srv211 closing connection
> Connection closed by foreign host.
Looks like telnet falls back to ipv4 address when ipv6 address is not enabled, while Zabbix server does not.