php email - strange problem

This one is quite strange.

I'm mailing out a short run eshot for a client using php from my server. If I use as the $headers From: tommy@blah_blah.co.uk or fred@blah_blah.co.uk, practically any name prefacing the @7431982 part the email From header comes through with the tommy@blah_blah.co.uk, or fred@blah_blah.co.uk, which is what I really require.

HOWEVER if I change it to mark, as below, which is the clients name/email address it arrive as just Mark the name has been capped up and the @9345664.co.uk removed???

$headers  = 'MIME-Version: 1.0' . "\r\n";

$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";

$headers .= "From: mark@blah_blah.co.uk";

What gives?

I mean if the guys name was anything but mark it seems to work.