Question
You receive the following message when attempting to send mail to a remote server.
Your message to mx.sweetwater.com was rejected.
I said:
EHLO mainserver.bitheadz.com
And mx.sweetwater.com responded with
550 Unknown command
Answer
This indicates that the remote server does not support the EHLO command. Further it has not returned a valid response for the failure so the message would not have been processed any further.
According to RFC2821, the remote server should reply with one of:
500 Syntax error, command unrecognized 502 Command not implemented
To explain…
RFC2821 tells us:
5yz – Permanent Negative Completion reply
The command was not accepted and the requested action did not occur. The sender-SMTP is discouraged from repeating
the exact request (in the same sequence). Even some “permanent” error conditions can be corrected, so the human user may want to direct the sender-SMTP to reinitiate the command sequence by direct action at some point in the future (e.g., after the spelling has been changed, or the user has altered the account status).x0z – Syntax
These replies refer to syntax errors, syntactically correct commands that don’t fit any functional category, and unimplemented or superfluous commands.x5z – Mail system
These replies indicate the status of the receiver mail system vis-a-vis the requested transfer or other mail system action.
This mail server is telling us that we have:
– a permanent negative completion reply
– this is generated by the mail system
So this is a permanent error which will never be resolved and it is an error generated by system configuration (so it could be an anti-spam filter for example).
Had the mail server responded correctly, NTMail would have identified a “syntax error” and proceeded on to use the HELO command and deliver the email.
Unfortunately, we can’t alter NTMail to work around this problem with the destination server, because the destination is too broken!
Looking at the destination mail server, it looks like they are using a firewall product which attempts to hide its identity by failing to comply with RFC821 on connection as well. Your best bet would be to contact the owner of the remote system and ask them to speak to the mail server vendor in order to get a patch installed.
Keywords:error codes SMTP 550 RFC821 821 HELO EHLO