Wrong. RFC2821 says:

4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO)

These commands are used to identify the SMTP client to the SMTP server. The argument field contains the fully-qualified domain name of the SMTP client if one is available. In situations in which the SMTP client system does not have a meaningful domain name (e.g., when its address is dynamically allocated and no reverse mapping record is available), the client SHOULD send an address literal (see section 4.1.3), optionally followed by information that will help to identify the client system. The SMTP server identifies itself to the SMTP client in the connection greeting reply and in the response to this command.

Cheers, Gavin

It specifically mentions that dynamic addresses are acceptable. It specifically states that there are alternatives (an address literal)

RFC 2821 also says: There are two exceptions to the rule requiring FQDNs:

- The domain name given in the EHLO command MUST BE either a primary host name (a domain name that resolves to an A RR) or, if the host has no name, an address literal as described in section 4.1.1.1.

So, it seems to me that the RFC compliant way to do this is to use the literal address in the HELO Meaning that those who reject this are the non RFC compliant ones...