what application are you sending mail with?
In PHP I use:
mail(TO, SUBJECT, MESSAGE, [OPTIONAL HEADER]) Like this:
mail("geir@linuxboxonline.net", "Test subject", "This is a test mail",
"From:
web@linuxboxonline.net\n" .
"Reply-To:
web@linuxboxonline.net\n" .
"X-Mailer: My own script.";
This sends a mail to me:
geir@linuxboxonline.net, but instead of showing an unvalid adress, I specify with From: ...\n and Reply-To: ...\n
The server I use has Sendmail as MTA.
The way I've written this, with \n at the end and first letter is capital is important for sendmail to recognize optional header.
Hope this can help.
-Geir O.