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I am setting up a custom .NET-based SMTP server. It is listening on port 25 and responding to HELO command with host name that I configured (it matches the host name resolved by reverse DNS lookup and also I have proper A and MX records in my DNS). The server seems to work well (I can send emails to gmail, yahoo, mail.com, zoho, outlook.com) and the HELO test passes with www.isnotspam.com. But mail-tester.com and allaboutspam.com both see wrong HELO response and warn me about it. That indicates there might be a problem with my server that some other email receiver servers might complain about. Below are some warnings (I replaced my IP and domain name with dummy values).

Mail-tester.com warning - notice the exclamation marks

Your IP address 10.10.10.10 is associated with the domain mail.yourdomain.com. Nevertheless your message appears to be sent from !10.10.10.10!.

Allaboutspam.com warning

Email Server HELO is <[10.10.10.10]>. HELO greeting does not contain a valid domain. We would recommend to have a valid domain in HELO greeting. Ideally HELO greeting should be the same as Email server host name (in FQDN format).

When I connect to my mail server with telnet and issue a HELO I get:

250 mail.yourdomain.com

What can be wrong here? Should I care about those warnings?

migrated from stackoverflow.com Dec 19 '17 at 23:31

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The problem turned out to be the HELO command issued by my server when sending the email to mail-tester.com - i.e. my server's response to mail-tester's HELO didn't really matter. I had misconfigured the object responsible for sending the emails - when using MailKit SmtpClient the key is to set this property http://www.mimekit.net/docs/html/P_MailKit_Net_Smtp_SmtpClient_LocalDomain.htm to proper host name (mail.yourdomain.com in my case).

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