How do I fix "501 Syntactically invalid HELO argument(s)"? - email

Question

I'm using exim on both the sending and relay hosts, the sending host seems to offer:
HELO foo_bar.example.com
Response:
501 Syntactically invalid HELO argument(s)

Answer 1

Possibly a problem with underscores in the hostname?
http://www.exim.org/lurker/message/20041124.113314.c44c83b2.en.html

Answer 2

Underscores aren't actually valid in internet host names, despite some people using them anyway. A sane DNS server should not allow you to have records for them.
Change your system's host name so it's valid, hopefully this will fix it.

Answer 3

After spending so many hours trying to fix this problem, which in my case just come up from nothing, I ended up with a solution. In my case, only the systems deployed to Suse OSs suddenly stopped sending emails but not those ( the same ) running on Ubuntu. After exhausting and eliminating all the suggested possibilities of this problem and even considering to change de OS of those machines, I found out that somehow the send email service is sensible to the hostname of the host machine. In the Ubuntu machines the file /etc/hosts have only the following line:
127.0.0.1 localhost
and so were the Suse machines, which stopped sending the emails. After editing the /etc/hosts from Suse machines to
127.0.0.1 localhost proplad
where proplad is the hostname of the machine, the errors were vanished. It seems that some security policy ( maybe from the smtp service ) uses the hostname information carried through the API, which was being ignored in the case of the Ubuntu machines, but not in the case of Suse machines. Hope this helps others, avoiding massive hours of research over the internet.

Answer 4

Diago's answer helped me solve the problem I have been trying to figure out.
Our Suse OS also stopped working out of nowhere. Tried every suggestion that I found here and on google. Nothing worked. Tried adding our domain to etc/hosts but that did not help.
Got the hostname of server with the hostname command. Added that hostname to the etc/hosts file just like Digao suggested.
127.0.0.1 localhost susetest
I saved the changes, then ran postfix stop, postfix start. And works like a charm now.

Answer 5

The argument to HELO should be a hostname or an IP address. foo_bar.example.com is neither an IP address nor a hostname (underscores are illegal in hostnames), so the error message is correct and there is nothing to fix.

Answer 6

Using qmail I came across this problem. I realised this was because of a previously unfinished installation.
1) When sending email qmail announces itself to other SMTP servers with "HELO ..." and then it adds what is in the file at: /var/qmail/control/me
(sometimes the file is located at /var/qmail/control/helohost)
2) This file should have a hostname with a valid DNS entry in.
Mine did not it had (none) which is why mails were failing to be sent.

Answer 7

I found that my local dev server suddenly stopped sending emails (using external SMTP) and on the server logs I found:
rejected EHLO from cpc96762-*******.net [..**.68]: syntactically invalid argument(s): 127.0.0.1:8888/app_dev.php
127.0.0.1:8888/app_dev.php is my local URL, I am using Docker, Symfony and Swift Mailer.
The only solution that helped in my case was adding the parameter:
local_domain = "localhost"
to my Swift Mailer configuration. That solved all the problems.
See the docs for the Swift Mailer local_domain option: https://symfony.com/doc/current/reference/configuration/swiftmailer.html#local-domain