5.5.1 501 Error, Can't Send Mail [closed]

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In the last 4 days, my mail server cant send mail to some domains. I receive these errors:

(abc (at) aygaz.com.tr), Result: failed, Status: 5.5.1 501 Syntax error in command parameter(s开发者_StackOverflow中文版) Domain name expected

(abc (at) total.com.tr), Result: failed, Status: 5.5.1 501 Syntax: HELO hostname

(abc (at) telemaks.com.tr), Result: failed, Status: 5.5.1 501 Syntactically invalid HELO argument(s)

(abc (at) ziraatbank.com.tr), Result: failed, Status: 5.5.1 501 Syntax error in parameters or arguments

(abc (at) normsan.com), Result: failed, Status: 5.5.1 501 5.5.1 HELO/EHLO requires domain address

I looked the log files for last 2 years, but find this problem only last 4 days. I searched the error code, but cant solve the problem. Error code descriptions tell same things. "Your email adress, domain invalid, character problem etc."

There is Kerio Mail Server 6.7 on Windows Server 2003.

I tried to send from different IP adresses, domains, web mail/outlook and disabled antivirus program on the server but problem can't solved. Mail server ip adresses aren't blocked by blacklists.

What is the reason of this problem, and how can I solve it?

Thanks.

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Typically, after connecting to an SMTP server, the first line of the request is simply:

HELO

It looks as if the SMTP server you're connecting to now expects the hostname as an argument:

HELO example.com

Make the change and retry.