PHP sending email contents as an attachment, not in the body

I'm using PHP to send emails. The problem I have is that when the emails reach a certain length, the emails are sent with the email contents as an attachment, not as part of the body. I've looked thru all the mailing lists, but I can't find a reference to a similar problem. Has anyone come across this before? Is there is any way to control this behaviour? Is there a way of forcing mail() or imap_mail() to include the contents in the email body? imap_mail ($testAddress, $title, $emailContents, "from:user@domain.org.uk"); mail ($testAddress, $title, $emailContents, "from:user@domain.org.uk"); Many thanks, Tom Pinder