embarqmail?

[Matt]

The best thing to do would be to complain to the embarq postmaster and tell them that they are rejecting mail from the null sender <> which makes their email systems non-RFC compliant.

We only accept mail from RFC compliant mailservers in attempts to curb spam delivery. I am not sure why some ISPs think they are immune to the RFC's but I have seen a few that think they are not required to follow the RFCs for one reason or another...

Per RFC 1123

5.2.9 Command Syntax: RFC-821 Section 4.1.2

The syntax shown in RFC-821 for the MAIL FROM: command omits
the case of an empty path: "MAIL FROM: <>" (see RFC-821 Page
15). An empty reverse path MUST be supported.

The specifics can be found in the SMTP RFC which is RFC 821
http://rfc.net/rfc821.html

On a side note, you can allow mail from these types of mail servers if you have a dedicated server but I don't advise it since you will see about a 50&#37; increase in the volume of spam that gets delivered because of it.