The error message "Invalid EHLO/HELO domain" is caused by an configuration error in the sending mail system: the error means, that in the opening of SMTP mail transfer connection (so called HELO/EHLO-handshake) the sending mail server gives, as its own name, a name that can not be found in the Internet domain name system (DNS). Being somewhat pedantic in this regard, the incoming mail server at jyu.fi considers that to be an attempted scam and refuses to accept any email.

Probably most often this error occurs if the sending mail server has not been properly configured to use the public name intended to be used in sending email. The mail server may then, for example, use the internal name of the server (or physical server name), that may not be a valid DNS name that can be found in the DNS, at least in the form given in the HELO/EHLO handshake: the server name may, for example, be just a first part of a proper DNS name without the domain part, and then the name may not be a valid DNS name outside of the sending organisation - even though it may be a valid name inside the organisation (where the domain part is appended to names by default). 

The internet standard (RFC) describing the internet mail transfer protocol (Simple Mail Transfer Protocol/SMTP) requires that in HELO/EHLO handshake the sending mail server should give its own name to the receiving mail server and if the name can't be found in the DNS, it can be justifiably said that the sending mail server has not given its own name! This requirement is stated in the SMTP standard in such a way (ie. "should give it's own name"), that in RFC parlance it means it should not be deviated from without especially good reasons and even then the deviator takes full responsibility of the consequences or any problems arising from deviating from the recommendations of the standard.

This error is a problem that will not be fixed by any other way than fixing it in the sending mail server, and as such it is the responsibility of the admins (ie. postmaster) of the sending mail server. The original sender of the failed email has no other means to affect the situation except to inform local mail admins of the problem or, of cource, by sending mail from another email service that does not have this name problem.

Quite likely, checks as stringent as this, are not performed in most internet organisations. In jyu.fi they are used to reduce the amount of SPAM email reaching jyu.fi. Every organisation respecting itself, should be expected to configure their mail servers properly also in this regard. This problem may very well not be encountered in sending mail into many (or even most) other organisations (besides jyu.fi). That fact does not, though, in any way alleviate or invalidate the fact that the sending mail server is improperly configured in and the configuration should be fixed.

With legitimate email this problem occurs perhaps a couple of times a year.