Recently I get the this error below when I testing SMTP 25 port on my email server and I want to share that issue with you: 220 ****************************
When I send email to external email server and vice versa all messages stayed in queue. After that I checked telnet smtp port from internal and external then I released that telnet session to our mail server on port 25 return that error above. Email server use Cisco as a firewall and mail flow happens over that. On Cisco firewall has a feature SMTP fixup and that was enabled. After disable that option all problems gone.
To fix that You must to login your firewall and select Configuration tab -> Firewall button, then select Service Policy Rules. Edit inspection_default policy and Disable ESMTP option.
After that I test my telnet and I could send helo and ehlo command and receive required response.
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GOOD LUCK!!!
