Just Thoughts

Sending from my company email to another company email I got this error (link)

smtp; 550 5.5.0 Invalid EHLO/HELO domain

It’s because the other end mail server complies with SMTP standard (RCF 5321)

For our external email transfers, our spam filter handles it, that is Mail Transfer Agent (MTA). The problem was the FQDN configured didn’t publically resolve. Our MTA public IP didn’t have a Host A record on public DNS. It’s just natted to a public IP through our firewall.

You might wonder how this is possible?

See we receive through one SMTP connector (SMPTin.company.com) and send throughout another (SMTPout.company.com). So our DNS records, MX records, A records, are pointing to our (SMTPin.company.com)

So the HELO/EHLO value our mail server provided in the mail session when connected to the other end was not a publically resolvable FQDN.  The value SMTPout.company.com, didn’t have an A record associated with it.

Updated the DNS records with the SMTPout.company.com A record.

I’m not sure how many mail servers out there comply with RFC5321 standard, but if you have this misconfiguration, you’ll face a problem when sending to such mail servers.

Thanks Nick for all the help you provided