AGB 2.3.1: sendmail "Data Format Error" for E-mail

Hi
I have E-mail notification turned on for "private webmaster messages"

Every time I use the Private Message feature, I get an error in my mail.log which says:

MSGID: to=me@mydomain.com, ctladdr=wwwrun (UID/GUID), delay=00:00:00, xdelay=00:00:00,
mailer=relay, pri=30115, relay=localhost.mydomain.com. [ip.ip.ip.ip], dsn=5.6.0, stat=Data format error

I have my php.ini set-up like this:
sendmail_path = "/usr/sbin/sendmail -t -i"

and I can resolve localhost.mydomain.com

Is this a sendmail issue?

Thanky for help
Rgds
Bunts